first published by Cassell, reprinted by Exbury Gardens, 1987, p. 35. PINE BARK AND AZALEAS This book is an invaluable source of information on Mr. Lionel de Larry Brown Rothschild's life and work, Exbury, Hammond, Louisiana and highly practical rhododendron culture. The book is a well-produced, hard-cover edition of 138 pages in- cluding 20 fine black-and-white illus- Use of pine bark in container culture of azaleas trations, plus 65 full-page color plates of plants in bloom. Peter Bar- ber had served as a major in the army As anyone who has grown ornamental plants in containers in the south- with Edmund de Rothschild, and eastern United States knows, milled pine bark is a great ingredient for a growth later become Managing Director of medium. Its qualities include good drainage and heat release, acidity, lack of Exbury Gardens. toxicity, and very slow decomposition rate. And anywhere that pine trees are harvested in quantity it has the additional advantage of wide availability and Berrisford, Judith. Rhododendrons and Azaleas. London: Faber and Faber, low cost. 1964, p. 45. The pine bark that I will be referring to throughout this article is described Berrisford, ibid. as being run once through a hammermill and at least six months old. This results in a range of particle sizes that is considerably coarser than bark that is Berrisford, Op. Cit., p. 46. usually sold as potting medium for greenhouse plants or houseplants. The Phillips and Barber, Op. Cit., p. 35. coarser bark is considered a necessity for growing healthy plants in containers in full sun in the South. If you are growing azaleas under less severe condi- In 1926 (Mr. Lionel) acquired most of tions, a finer bark may be more desirable. the collection of Thomas Lowinsky, one of the leading amateurs of the day and a RHS gold medallist, whose Azaleas can grow very well in 100% pine bark. However, one drawback to fine garden at Tittenhurst, such use is the low water holding capacity of pure bark. Another property, Sunninghill, was celebrated also for which can be either a plus or a minus, is its light weight. For shipping, light some of the rarer forms of conifer, weight is desirable, but plants in pure bark are easily blown over, such as the curious pendulous cedars and the very odd weeping wellingtonias, or 'ghost trees'," Plants grown in 100% bark or very coarse bark may also have difficulty in Phillips and Barber, Op. Cit., p. 26. becoming established when planted out, especially into heavy soils. The roots can easily dry out before they grow beyond the original container volume, (7) Year Book of the Rhododendron Associa- even if the surrounding soil is moist enough. Addition of up to 10% sand to tion, 1934: pp. 113-114. the container medium helps overcome these problems. (8) Phillips and Barber, Op. Cit., p. 20. Several years ago I compared the growth of various plants in media contain- (9) Communiqe from Mr. Nicholas de ing pine bark, 10% sand, and Canadian peat in percentages ranging from 0 to Rothschild, 9 October 1996. 90%. The results of these tests are shown in Table 1 (1). (10) Letter from William C. Miller III, 12 shown as May 1996. Results obtained in two separate tests in consecutive years are "Test I" and "Test II". (11) Exbury Gardens, [Christina Dykes and Charles Orr-Ewing, produced by TABLE 1 Exbury Gardens.] "An Introduction." (12) Communiqe from Mr. Lionel de Weight of prunings from 'Carror' azalea plants grown in various media Rothschild, 10 June 1996. and pruned at 4" height after one season of growth.(1) PERCENTAGES FRESH WEIGHT (g.) Jaacquelyn Kuehn writes from her home in western Pennsylvania, where she is eagerly anticipating the first blossoming of her young PEAT PINE BARK SAND TEST I TEST II deciduous azaleas this spring. She is currently the editor-in-chief of Pomona, journal of the 0 90 10 33 63 North American Fruit Explorers (NAFEX). 5 85 10 52 73 10 80 10 55 84 Photographs by the author 20 70 10 55 84 Jacquelyn A. Kuehn Box 29 Lucernemines, 40 50 10 68 92 PA 15754 PHONE: (412) 479-0266 First 90 0 10 76 100 North American Rights ❑ 8 THE AZALEAN/March 1997 Under the conditions of these tests, were 18" in diameter and three gallons The research that I will use to sug- growth and quality of azaleas increased of bark per plant were used for treat- gest how pine bark might be used for with each increase in peat percentage. ments two through five. Peat was used Since cost also increases with increas- azalea planting was not done with aza- at one and one-half gallons per plant ing peat, use of 10% peat was prob- leas as the test plant. Blueberry in order for the cost of the materials to (Vaccinium ashei) was used instead. It ably a good compromise between best be more nearly equal. may seem like quite a leap to compare growth and lowest cost. If cost is not blueberries and azaleas, but actually • a major consideration, use more peat. they are closely related, both belong- TABLE 2 I must hasten to add that another ing to the heath family (Ericaceae). Both Effects of pine bark on growth of characteristic of pine bark is its inertness. have a fine and slow-developing root blueberry plants in the field Its decomposition rate is so slow that system and their cultural requirements are quite similar. it makes negligible amounts of essen- TREATMENT TOP ROOT tial nutrients available to the plant. RATING' RATING' These elements (nitrogen, phospho- My hypothesis for this work was based on the previous explanation that rous, potassium, calcium, magnesium, (1) No bark 1.6 0.8 sulfur, iron, manganese, copper, zinc, bark is not considered to be a good (2) Bark on surface boron, and molybdenum) must all be material for incorporation, but is a (mulch) 3.5 2.4 good medium by itself. In a prelimi- added either by incorporation in the (3) Bark layer 1" growth medium, application on the nary test, holes 15" in diameter and below surface 3.4 4.6 surface, dissolved in the irrigation wa- six inches deep were dug in a fine (4) Bark layer 3" sandy loam field soil with a low ter, or a combination of these meth- below surface 3.6 4.2 ods. There is an almost infinite num- organic matter content. Blueberry (5) Bark mixed ber of fertilizer combinations that can plants from three-quart containers with soil 2.3 1.8 were planted in these holes with one be satisfactory. and one-half gallons of bark placed as ' Rated on a scale of 0 - 10: 0 = shown in Table 2. Additional expla- Use of pine bark in landscape planting dead, 10 = excellent quality nation may be needed for the placement in treatments three and four: the bark Pine bark is widely used as a mulch- 2 Rated on a scale of 0 - 10: 0 .- roots was placed in a layer extending from ing material and is very effective not extending beyond original pot because of the same properties that the root ball of the plant to the sides medium, 10 = roots extending to ex- of the hole. make it a good container medium in- tremities of planting hole. gredient. If weed control is a primary reason for mulching, only very large A year after planting, the plants bark particles should be used; other- were rated for quality of top growth. The only records taken in this test wise weeds can grow right in the Then they were carefully dug and root were yield of berries, which generally mulch. development beyond the original appeared to reflect plant size. The first rootball was evaluated. The average year yield was highest with bark lay- Compared to other organic materi- ratings are shown in Table 2. ered three inches deep and lowest als, bark is considered a relatively poor (U n published data, Hammond Res ea rc h with both incorporated materials. The Station.) material to incorporate into a land- second year yields followed a similar scape soil. Except for very fine par- pattern except that incorporated peat ticles, bark acts somewhat like an equal These ratings tend to indicate that was alone as the poorest treatment. the layered bark (including that on the amount of gravel when mixed with a Heavy production began the third year mineral soil. surface) improved top growth the first and all treatments utilizing pine bark year after planting and that the layers (including incorporated bark) yielded one inch and three inches deep greatly Theoretically, a bark and soil mix- much more than incorporated peat, ture would have to be at least 80% bark improved root growth during that first which was only slightly better than the (and therefore not more than 20% soil) year. The hypothesis that roots would control treatment with no additive. to be a medium that provides better grow throughout the bark layers much aeration than the soil alone. This is faster than into the surrounding soil Well, that seems to refute what I because the large pore spaces (i.e., the was borne out. said earlier about bark not being a good spaces that are filled with air after irri- material for incorporation. Perhaps gation water drains away) are no more After this preliminary test, a larger tests that so indicated were of too short planting was made using the same basic than 25% of the total volume of bark. a duration and, perhaps, during the If all of these large pore spaces are treatments plus one with incorporated second year of my test, bark began to filled with soil, aeration is poor.