Understanding the Turfgrass Crown

A PRACTICAL RESEARCH DIGEST FOR TURF MANAGERS

Volume 9, Issue 5 • May 2000

BACK-TO-BASICS IN THIS ISSUE

• Understanding the . Understanding the Turfgrass Crown ... 1 Tlirfgrass Crown Mother of a turfgrass Leaf and bud emergence By Richard J. Hull Stems emerging from the crown his is the first of a three-part series on turfgrass morphology, function, physiology and management implications. This series will feature crowns, leaves and roots, all vitally Rhizome growth Timportant parts of turfgrass plants. Today we will concentrate on turfgrass crowns. In his recent textbook on turfgrass management (1998), Nick Christians states, "The Stolons' horizontal growth crown is the center of activi- Flowering culm ty for the turfgrass plant, and as long as it is alive the plant YOUNG LEAF Crowns as storage organs is alive." If anything, this is an FLOWERING CULM Turf management and . NODES understatement. The crown crown functions literally is the turfgrass plant, TILLER NODES • Why it T^kes IO Years or at least that which makes a to Bring Products to turfgrass a perennial plant. Market...... 10 The primary growing points RHIZOME AND/OR STOLON NODES (meristems) are located in • Vermiculite Scare the crown which means all Worrisome ..... 14 grass organs originate from • 16-Minute Checklist. 15 the crown. Most of the energy reserves of a turfgrass plant are stored in its crown. During the winter and even during drought-induced summer dormancy, the crown may be the only part of a grass plant that survives. Considering all this, it may appear strange that most turf folks rarely think about the well-being of grass crowns Figure 1. Stylized image of a turfgrass croum with nodes when developing a turf man- identified that will contribute to a flowering culm, agement program. produce tillers, and give rise to rhizomes or stolons. Only Leaves are certainly con- the flowering culm internodes unll elongate. Roots are sidered when deciding on a much reduced for a croum old enough to have so many height of cut and the impor- nodes. All but the youngest leaf are removed. Visit us at www. landscapegroup. com TURFGRASS TRENDS

Executive Editor Sue Gibson 440/891-2729; 440/891-2675 (fax) the adventitious roots which emerge from sgibson@advanstar. com tance of roots is generally acknowledged when scheduling fertilizer applications or crown nodes. The primary roots can survive Managing Editor Curt Harler aerification. Crowns are down there where for several weeks or more than a month and 440/238-4556; 440/238-4116 the leaves meet the roots and you do not make a modest contribution to the plant's [email protected] want to scalp them by mowing too close, nutrition and supply of water. Eventually, Senior Science Editor but otherwise they are not worth much they are shed and the adventitious roots Dr. Karl Danneberger attention. Today I hope to change that atti- assume the task of establishing and main- Consulting Editor taining an intimate relationship with the Chris Sann tude. soil. Root structure and function will be the Group Editor Vern Henry subject of a future report. The mother of a turfgrass For the turf manager, the most impor- Production Manager Karen Lenzen The crown is a stem from which all tant and obvious function of turfgrass 218/723-9129; 218/723-9576 (fax) other stems, leaves and most roots originate. crowns is their role in initiating leaf and klenzen@advanstar. com It is literally the mother of a grass plant. As stem development. The apical meristem is Circulation Manager positioned at the apex of the crown where Frank Christopherson long as the crown is alive and healthy the 218/723-9271 plant can survive even when all other it undergoes cell divisions, producing new

Croup Publisher organs have been killed. Insects and diseases cells for leaf and nodal bud development. John D. Payne that spare the crown, even if they ravage Unlike apical meristems of broad-leaved 440/891-2786; 440/891-2675 (fax) leaves and roots, rarely cause turfgrass plants (dicotyledons), the apical meristem jpayne@advanstar. com death. Diseases such as Pythium blight and in grasses produces leaf primordia that Corporate & Editorial Office 7500 Old Oak Blvd. severe infections of stripe smut will attack retain meristematic activity at their base, Cleveland, OH 44130-3369 the crown and, in so doing, kill the plant. below the shoot apex. Mew Subscriptions The crown originates from the basal 888/527-7008 nodes of a seedling grass plant (Hull, Abstracts: 800/466-8443 1999a). Because most early shoot growth of Cell divisions Reprint: 440/891-2744 grasses involves the initiation and elonga- The cell divisions occur in a line around Permission: 440/891-2742 Single copy or back issues: tion of leaves, the stem remains just below the apical dome, giving rise to a leaf pri- Subscription/Customer Service the soil surface and experiences little verti- mordium that forms a ridge just above 888/527-7008; (fax) 218/723-9437 cal growth. This continues for some time what will become a stem node. Cells at the Web site address: www. landscapegroup. com while leaves are being produced and a root base of the leaf primordium remain meris- system develops. Both leaves and roots orig- tematic and form the intercalary meristem inate from the crown but the crown itself at the base of each developing leaf that sup- grows little except for a modest increase in ports the continued growth of that leaf W diameter (Fig. 1). ADVANSTAR even after more apical leaves have been ini- COMMUNICATIONS Leaves are produced as nodes emerge at tiated.

Chairman and the stem apex but there is virtually no At any given time, most turfgrasses have Chief Executive Officer internode elongation. As with all flowering from five to 10 leaf primordia present, Robert L. Krakoff plants, a bud is formed just below the line below the apical meristem at various stages Vice Chairman of attachment between each leaf and the of development (Turgeon, 1999). Further James M. Alic stem but on the side opposite the point of leaf development from its primordium and VP-Finance, CFO & Secretary leaf initiation. Because the stem does not David W. Montgomery the phytomer concept of grass structure elongate, these buds are clustered on alter- will be discussed in a future article devoted Executive Vice President, Business Development nating sides of the crown. Such buds do not to turfgrass leaves. Skip Farber resume growth into basal shoots until the A nodal bud is initiated at the time Executive Vice Presidents plant achieves the appropriate develop- when the ring of dividing cells that will William J. Cooke mental state. become a leaf primordium completely Alexander S. DeBarr Morris R. Levitt The basal nodes of a developing crown encircles the shoot apex. A small group of Daniel M. Phillips give rise to the adventitious roots that will surface cells just below the developing leaf Vice President & General become the principal root system of the primordium and on the side opposite leaf Counsel Eric I. Lisman plant. The primary roots emerge from the initiation, begin to divide and form a new germinating seed but they are replaced by meristem. Treasurer and Controller Adele D. Hartwick This meristem develops into a nodal promote tillering. Excessive nitrogen pro- bud and has the potential of becoming a motes leaf growth but depresses tiller pro- secondary apical meristem capable of production. ducing a secondary shoot: tiller, rhizome or Tiller production is not unlimited. stolon (Fig. 1). There can be no more tillers than there are Nodal buds are formed opposite most crown buds that can develop into tillers. grass leaves; exceptions being the seed leaf With one bud per node, the number of (coleoptile), the bud scale (prophyll) and nodes in a grass crown sets the upper limit the flower scale (palea). These are all first for tillers that can be produced. leaves produced by an apical meristem of a Of course, as each new shoot (Madison, 1971) and generally leaf is initiated from arise from a node that fails to initiate a bud. the crown apex, a new Most tillering OCCUTS iïl bud is formed and 7 7 thus another potential COOlseUSOU gTOSSeS during Stems emerging from the tiller. Most grasses can the autumn and is fUOSt crown produce no more than Secondary stems can emerge from nodal 14 to 20 leaves per abundant when tempera- buds on the crown and contribute enor- stem (Madison, tures are moderate to low. mously to the ability of a grass plant to pro- 1971). duce a thick stand and spread vegetatively. Not all crown These secondary stems include tillers, rhi- buds develop into secondary shoots — zomes and stolons. some remain dormant and never resume Tillers emerge from nodal buds near the growth. Stand density influences the degree midregion of a grass crown. The buds nor- of grass tillering. More tillers per plant are mally resume growth while surrounding observed in thinly spaced plants than in leaves are still functional. Growth is posi- dense stands of grass. tively geotropic so the tiller elongates The base of each tiller develops into a upward between the sheaths of leaves. secondary crown, which remains attached Tillers are said to be intravaginal shoots to the original primary crown. As several reflecting their growth among and emer- tillers emerge from crown buds, the total gence from encircling leaf sheaths. Tillering crown structure becomes increasingly com- adds shoots to a turfgrass stand and con- plex and can enlarge substantially. The fre- tributes in a major way to increased turf density. Crown buds commence growth as tillers when the plant reaches a specific age or more likely a suitable energy status. Most tillering occurs in cool-season grasses during the autumn and is most abundant when temperatures are moderate to low Tillering also occurs in the spring when temperature appears to be less of a factor. During the hot summer months, tiller emergence is much reduced although this can be influenced by management. In general, mowing promotes tiller production and this is also influenced by leaf length. Long leaf blades are correlated with few tillers while shorter leaves are associated with more frequent tillering (Etter in Madi- son, 1971). Very close mowing reduces the photosynthetic leaf surface so much that the plant's energy status is lowered and Figure 2. Impact of nitrogen supply on rhizome grourth of tillering declines. Modest fertilization will Kentucky bluegrass (Madison (1971). quent and close mowing associated with is favored during September through turfgrass management limits secondary November, declining again with the onset crown development to levels well below of cold weather. Managing grass as turf that observed in unmowed or infrequently appears not to inhibit rhizome initiation cut perennial grasses. and growth although the length of rhizomes is normally much reduced. This is likely a restriction imposed by stand densi- Rhizome growth ty rather than by insufficient energy. Rhizomes are Rhizome growth progresses through horizontal shoots that three phases. The initial primary growth A rhizome is a stem also emerge from occurs in a downward direction that places basal crown buds in the apex well below the soil surface. Soon, distinctly different from some grasses. Rhi- this is followed by horizontally oriented the crown in that internodes zome initiation occurs secondary growth that is the major growth in a seedling plant, phase of most rhizomes. Unless stand elongate by as much as tiller or plant originat- crowding is limiting, the secondary phase an inch or more. ing from a rhizome or can consist of 20 nodes being produced stolon bud at about over a distance of several feet. the five-leaf stage. The A rhizome is a stem distinctly different trigger for rhizome initiation appears to be from the crown in that internodes elongate carbohydrate or energy status (Madison, by as much as an inch or more. The rhi- 1971) but we cannot exclude a possible zome apical meristem is covered by scale role for day-length or temperature. leaves called cataphylls, one produced at Little rhizome growth occurs during each node. the winter in cool climates but increases As internodes elongate, the nodes and markedly from March through June. The their cataphylls are arrayed in regular inter- high temperatures of summer depress rhi- vals along the rhizome length. At each zome growth in cool-season grasses but can node, opposite the cataphyll, is a bud which stimulate their growth in warm-season normally remains dormant until induced to grasses. resume growth and produce a new plant. Again, in cool climates, rhizome growth Eventually, the rhizome enters its tertiary growth stage when the tip grows upward and the cataphylls, that until now have consisted only of a leaf sheath, develop a collar and form a rudimentary blade. When the cataphylls covering the rhizome apex break through the soil and are exposed to light, internode elongation stops and a new crown develops. The former rhizome, now crown apex, continues producing leaf primordia that develop into full grass leaves. The crown functions much as did that of the parent plant from which the rhizome originated. Low rates of nitrogen fertilization will stimulate rhizome growth but even moderate rates will retard growth (Fig. 2). Nitro- 23 MAY 25 JULY 1 6 OCT gen also will exacerbate the inhibitory Figure 3. Seasonal changes in simple sugar content of croum effect of high temperatures on rhizome tissue from Kentucky bluegrass groum at two N levels (Hull & growth. At 90-100°F, nitrogen applications Smith 1974). can cause rhizome death in cool-season grasses. Reduced rhizome growth in zome or stolon growth begins, several nor- response to nitrogen application results mally are initiated within a short time from from fewer nodes being initiated rather basal crown buds, allowing the plant to than shortened internode elongation. spread in all directions. Nitrogen can promote rhizome growth In a dense sod situation, rhizome and under some circumstances. While only low stolon growth is usually restricted but has nitrogen plants will initiate many rhizomes great potential to fill- during the fall, autumn fertilized plants in open areas caused have been observed to produce more rhi- by disease or mechan- T r . 1 j •, , . zomes during the following spring. . i . . t~ i i .If stand density is not too i J J Again, much of this nitrogen effect can ical injury. Early rhi- be attributed, at least in part, to plant ener- zome and stolon great, stolons can grow for growth knits a turf- i r . i r .1 - gy status. This nitrogen inhibition on rhi- 6 zome initiation and growth is more charac- j . . i several feet before their apex grass sod together, J J R teristic of mature turf and much less making it possible to turns upward and initiates evident in young turfgrass stands. harvest sod within 8 to , . r / . r 1*1 r. i a terminal snoot from which 12 months alter seed- J Stolons' horizontal growth ing. a new crown will develop. Stolons are similar to rhizomes in that A turfgrass crown they, too, arise from basal buds on the turf- will be induced to grass crown. They differ in not undergoing flower when conditions are favorable. In a downward primary growth phase but cool-season grasses, flower induction fre- rather growing horizontally right from the quently involves exposure to cold temper- beginning. atures for a period of several weeks or As a result, stolons grow along the soil longer. This vernalization response is cen- surface and their cataphylls are more likely tered in the crown's apical meristem but to consist of both a leaf sheath and small does not elicit an immediate response, blade. Also, the nodal buds of stolons are probably because it occurs during late fall more likely to initiate new plants quickly and early winter. Temperatures of between than are the buds of rhizomes. If stand den- 32°F and 45°F are sufficiently cold for ver- sity is not too great, stolons can grow for nalization, although the process is more several feet before their apex turns upward rapid and complete at lower temperatures. and initiates a terminal shoot from which a A period of warm temperatures can dever- new crown will develop. nalize a plant (Turgeon, 1999, p. 36), Vegetative spread via stolons is charac- requiring a re-exposure to cold. teristic of many warm-season turfgrasses Normally, flower initiation also requires including bermudagrass, buffalograss, zoysi- an appropriate photoperiod (length of the agrass, carpetgrass, centipedegrass and St. day-night cycle). Vernalization predisposes Augustinegrass. Many of these grasses are the plant to flower but, for many cool-sea- commercially propagated and established son grasses, short days (long nights) are also by planting stolons. required for full flower induction. Among cool-season grasses, creeping Flowering hormone bentgrass, velvet bentgrass and rough blue- The photoperiodic signal is perceived by grass are the only ones that spread primari- the leaves, where it triggers the synthesis of ly by stolons. Cool-season grasses that a flowering stimulus. This 'flowering hor- spread vegetatively are more likely to do so mone' is translocated via the phloem to the via rhizomes. shoot apex where it induces flowering. The Both rhizomes and stolons arise from flowering hormone has not been identified buds at the base of a grass crown (Fig. 1). If but recent discoveries of the broad range of leaf sheaths are still present when a basal signal molecules that can be transported by bud resumes growth, the rhizome or stolon the phloem considerably expands the num- breaks through the sheath forming what is ber of possible candidates (Hull, 1999b). termed an extravaginal shoot. Once rhi- While most cool-season grasses are induced to flower during the short days of once flowering and seed set are completed. late fall, flowering is delayed until the long However, the nodes lower on the crown days and warm temperatures of spring. For that did not experience internode elonga- this reason, tillers initiated during the tion continue to produce tillers from their spring often will not flower until the fol- buds, and these do not die because the lowing spring while tillers produced during crown apex has flowered. In this way, the the fall will flower that spring. While crown enlarges as more secondary crowns December tillers might flower, tillers are added to it, which, in turn, will produce maturing in late Janu- tertiary tillers and crowns. ary or February did The grass crown is the only not flower (Madison, Crowns as storage organs truly perennial organ of a 1971, p. 32). When a crown The grass crown is the only truly peren- grass plant. apex is induced and nial organ of a grass plant. Roots and leaves initiated to flower, the normally live for less than a year. Rhizomes apical meristem elongates and produces and stolons are regenerated each year, many buds that will develop into spikelets which leaves only the parent crown with (flower clusters). At the same time, or soon any permanency. thereafter, the upper internodes of the Consequently, only the crown is able to crown begin elongating and lift the devel- maintain energy reserves to power new oping inflorescence through the tube growth and replace injured organs. Rhi- formed by encircling leaf sheaths. zomes, stolons and leaf sheaths do serve as During this elongation process, the temporary storage organs but their energy youngest leaves emerge as culm (stem) reserves are pretty much directed toward leaves and the maturing inflorescence is lift- local needs. Rhizomes and stolons support ed above the flag leaf into the atmosphere. new shoot and nodal root growth and rhi- This is the only time a grass crown apex is zome extension. Leaf sheaths support the visible as a true stem. Flower induction is ter- emerging inflorescence while roots actual- minal for the flowering culm — it will die ly serve a minor storage role in grasses. When you read about carbohydrate reserves accumulating or being used in response to a turf management practice, it is crown reserves that are most involved. Crown reserves undergo seasonal cycles being drawn down during spring and fall regrowth and replenished during the late spring and fall-winter periods. Yes, even during the winter, cool-season turfgrasses carry on photosynthesis that powers root growth and replenishes carbohydrate reserves in the crown (Hull, 1996). The turf may appear brown and nonfunctional but many green leaves lie just below the dead surface leaves and they are photo- synthetically active whenever temperatures are above freezing. Cool-season grasses store mostly simple sugars and fructans (Hull and Smith, Figure 4. Seasonal changes in fructan content of croum tissue 1974). Some temporary starch accumula- from Kentucky bluegrass groum at two N levels (Hull & Smith tion can occur in leaves, but the primary 1974). storage carbohydrates are fructans, which are polymers of fructose built on a sucrose Turf management and molecule. crown functions Both simple sugar and fructan concen- The turf manager is confronted with an trations undergo substantial seasonal varia- interesting problem in that the manage- tion, and this variation is strongly influ- ment program should be geared to preserv- enced by nitrogen nutrition (Figs. 3 & 4). ing the integrity of a grass organ that is Nitrogen stimulates shoot growth dur- largely invisible. ing the spring and this draws down all car- Crowns must be preserved because all bohydrate reserves to very low levels. Less other organs can be replaced except for heavily fertilized turf also experiences a them. Simply knowing the crown is present spring carbohydrate decline but it is much and is important represents a good first step less dramatic. During the summer and into to sound turf management. With this real- the fall, fructan levels gradually increase but ization, several management variables can they are always more abundant in low fer- be analyzed for their impact on the vitality tility turf (Fig. 4). of grass crowns. Simple sugars are present in lesser Mowing. Since all new leaves emerge amounts and they increase when metabol- from the grass crown, any mechanical ic activity is high (rapid growth or stress injury to the crown should be avoided. If induced respiration). For that reason, heav- grass appears to have heaved during the ily fertilized grass that is growing more winter or in any way appears excessively rapidly normally has higher simple sugar exposed, consider levels, except during the spring when ener- rolling or topdressing gy levels are generally diminished (Fig. 3). before the first mow- The turf may appear brown During the winter months, reserve carbo- ing. Raising the height r r . 77 and hydrates slowly increase unless conditions of cut for the first nonfunctional but many are such that photosynthesis is inhibited. mowings in the spring green leaves lie just below Warm-season grasses store starch in may reduce crown .1 1 ] r 1 j their crown and stem tissues along with injury when the cut- the dead SUTtaCe ieaVeS and variable amounts of sucrose. They also ting height is lowered experience seasonal variations with a are photo synthetically active spring-time low and a progressive increase lateFlowering is not a whenever temperatures are throughout the summer and early fall. problem for most above freezing These grasses generally experience winter grasses maintained as dormancy and carry on little photosynthe- a turf. However, Poa sis at that time. annua and some Kentucky bluegrass culti- Consequently, carbohydrate reserves are var s can flower heavily in the spring and highest in mid-autumn and slowly decline early summer. The flowering culms will die, during the winter and early spring. In mild but that is no reason to lower cutting height climates, where winter dormancy is not to remove as much dead stem tissue as pos- induced, warm-season grasses will grow all sible. year and their carbohydrate reserves nor- The crown below each flowering culm mally experience less seasonal variation. is not dead and will produce tillers from its For all grasses, low carbohydrate con- buds that will rapidly replace the lost grass. centrations in crown tissues indicate vul- Close mowing may scalp the crowns, caus- nerability to adverse environmental condi- ing their death, and it will surely reduce the tions and damaging management practices. leaf surface feeding the crowns, thereby At such times, the grass simply lacks the limiting their ability to generate new tillers. energy to respond aggressively to stresses or Remember, crown reserves may be low injury. Recognizing this fact may assist in during early to mid-spring. altering management practices so as to min- Fall is a good time to raise the cutting imize grass injury. height. Photosynthetic leaf surface is the greatest resource a grass plant has to replen- ish depleted carbohydrate reserves. Replac- ing roots and generating tillers in the fall can lower energy reserves in the grass crowns. the ability of grass to spread, repair damage These need to be replaced, and promoting and increase density. This is most likely to photosynthesis is the only way to do it. result from heavy nitrogen applications A higher cutting height will maintain a during mid-fall and mid-spring. larger leaf surface area which will make Fertilizer applied during early spring more carbohydrates available to the will generally be beneficial. If shoot growth crowns. During the winter, surface leaves has not yet commenced, nitrogen will not will desiccate and die stimulate excessive growth but will be but the health of your absorbed, reduced and assimilated and then r^ . i . r turf depends on the be available to support all growth when it is initiated. During the winter, surface ph0tosynthetic activity Other plant nutrients are less sensitive leaves will desiccate and die of lower leaves, if grass i i i i i r r is mowed too close, with respect to growth stimulation and the maintenance of adequate energy reserves in but the health of your turf there will be no lower crowns and other stems. Following a period depends on the photosyn- leaves>the stand wil1 be of rapid shoot growth and heavy clipping i . . . r t weakened and the turf production, whether stimulated by nitro- thetic activity of lower will be m prepared to gen or not, a light application of phospho- leaves. resume spring shoot rus, potassium and even micronutrients is growth. often beneficial. Fertilizer manage- Nutrients are lost from turfgrasses when ment. Carbohydrate clipping removal is heavy. These nutrients reserves in grass crowns will be reduced should be replaced before the plants are during the spring, and nitrogen fertilizer asked to tolerate stress conditions and increases this problem. Consequently, mid- recover from wear or other injuries. A spring fertilization that will stimulate major advantage of providing nutrients fol- excessive leaf growth should be avoided. lowing heavy growth is that they will Some of that growth is occurring at the increase the efficiency of metabolic expense of reserves in the crowns. processes, making more energy available for This is more than a problem of having crown replenishment and appropriate to mow more frequently or disposing of responses to stress conditions. additional clippings. Crown starvation can Water management. Crowns are at or be occurring, and that could result in stand just below the soil surface and, as such, collapse should disease or other stress con- have little insulation from rapid changes in ditions develop. Assimilating nitrogen is an temperature, moisture status and other energy demanding process, and if all it environmental variables. Crowns are also accomplishes is more rapid leaf growth that living stems that require oxygen for normal is soon removed by mowing, the grass functioning. plants suffer a net energy drain. Consequently, they should not be sub- Because nitrogen fertilization promotes jected to prolonged flooding or ice cover. leaf growth, it should be avoided whenever Other plant organs are no less sensitive but tillers, rhizomes or stolons are being initiat- can be replaced; crowns cannot. ed. A light application of nitrogen in early Heavy irrigation that promotes standing fall will probably stimulate tillering, and water or poor drainage that allows water to late fall fertilization after most tillering has pond for extended periods should be avoid- occurred also will be beneficial. ed. This is important during the warm Inappropriate nitrogen applications can months because metabolism is rapid and divert so much energy from rhizomes and oxygen supplies can quickly become stolons that they succumb to disease or are exhausted. killed by other stresses. Poorly timed fertil- It is no less important during the cold izer applications can seriously retard rhi- season because crown activity continues. zome and stolon growth and thereby limit Chapter 1. Anatomy and morphology of the turf- Prolonged periods of flooding can cause sig- grass plant, p. 1-48. Van Nostrand Reinhold Co., nificant injury due to anaerobic conditions New York. 420 pages. and disturbed metabolism. Disease and other secondary stresses can Turgeon, A.J. 1999. Turfgrass Management, Fifth be especially damaging to grass weakened Edition.; Chapter 2. Growth and development, p. by periods of anoxia. Soils are generally 15-47. Prentice Hall, Upper Saddle River, NJ. 392 wetter during the cold season and surface pages. flooding can easily occur without notice. This can even occur under a snow cover where it is especially difficult to monitor. Being conscious of good drainage is obviously an important part of sound turfgrass management. However, the sensitivi- ty of perennial grass crowns to anoxic conditions is another critical reason to take drainage problems seriously. It is not un- common for winter-spring disease problems to be aggravated by a weakened turf resulting from periods of surface flooding and anoxic stress. Hopefully this discussion has made you more conscious of the need for managing turfgrasses with crown well-being in mind. Crowns may not be visible but maintaining their health is an essential objective of effective turfgrass management.

— Richard Hull is Professor of Plant Science at the University of Rhode Island, Kingston, RI.

REFERENCES CITED Christians, N. E. 1998. Fundamentals of Turfgrass Management; Chapter 2. Introduction to the grasses, p. 9-31. Ann Arbor Press, Chelsea, Ml. 301 pages.

Hull, R.J. 1996. Turfgrasses. Chapter 33:781-796 IN E. Zamski and A.A. Schaffer (eds.). Photoassimilate Distribution in Plants and Crops: Source-Sink Relationships. Marcel Dekker, Inc., New York. Hull, R.J. 1999a. Back to basics: How turfgrasses grow. TurfGrass Trends 8(5):7-12. Hull, R.J. 1999b. How does turfgrass allocate energy? Golf Course Management 67(8):60-63.

Hull, R.J. and L.M. Smith. 1974. Photosynthate translocation and metabolism in Kentucky bluegrass turf as a function of fertility, p. 186-195 IN E.C. Roberts (Ed.) Proceedings 2nd International Turfgrass Res. Conf., Amer. Soc. Agron., Madison, Wl. Madison, J.H. 1971. Principles of Turfgrass Culture;