Landscape Basics: Success with Herbaceous Perennials
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Landscape Basics: Success with Herbaceous Perennials Bodie Pennisi, Paul Thomas and Sheri Dorn Department of Horticulture hether in a commercial installation or residen- fewer pest problems, require less water and pruning, Wtial garden, perennial plants can be successfully and have an extended flowering time. This information used to offer more landscaping choices, distinguish can be acquired from books, annual trade confer- your firm from the competition and create a niche for ences and Extension publications (refer to “Additional your landscape business. Perennial plants are complex, Resources” at the end of this publication). An excellent and it is best to contract or hire a professional land- way to see how new selections perform is to visit the scape architect for the design phase and train knowl- University of Georgia Trial Gardens located on campus edgeable staff in proper maintenance later on. in Athens (http://ugatrial.hort.uga.edu/). With a rich history of plant introductions, hybridiza- This publication is intended to provide the basics of tion and selection, we can now enjoy plants in Georgia perennial plant biology, ideas on design and installa- that until recently were once found only on a remote tion, and information on cultivation and maintenance mountain slope in Asia or a tropical rainforest in South of perennial beds. It should also serve as a quick guide America. The perennial plant trade offers more than for the most common and recommended perennials 3,600 species and cultivated varieties, and more are for Georgia. Common-sense tips from a professional added each year. With such an extensive palette, how landscaper’s perspective are also included. do you choose the right plant? Our advice is to follow three simple rules: Biology of Perennials The term “perennial” in the broadest sense means 1. stick to species and selections that are known to do plants that live for more than two years. This broad well in the area; the “bread-and-butter” plants; definition includes shrubs and trees that retain woody 2. match the optimum growing requirements; plant stems above ground in winter, including ferns, cacti, the “right plant in the right place”; and succulents, grasses, bulbs, some herbs and groundcov- 3. spend the extra time to maintain each species at ers. For the home gardener, however, it means a plant its peak performance (e.g., additional nutrition, with stems that usually die back in winter and a root mid-summer trimming, regular deadheading); go system from which new foliage and flowers grow the the extra mile. following year. Most perennials have herbaceous stems (e.g., Coreopsis, Achillea, Echinacea, Liatris), but a few You don’t have to plant 50 species in one landscape, or develop woody stems (e.g., hardy hibiscus, rosemary). always use the latest and most fashionable cultivars – There is also a category of plants, called “tender peren- you could make a stunning display with five species or nials,” which are annuals that may over-winter occa- varieties that have been in the trade for 20 years. That sionally, depending on how severe the winter is (Figure said, you should strive to familiarize yourself with new 1). A true perennial has a capacity to survive from one hybrids that have improved tolerance to shade/sun, year to the next. Figure 1. Colocasia (elephant ear; front left, heart-shaped foliage), a tender perennial, is shown here emerging from the ground along with Hosta, a perennial (back of bed, gray foliage). Tender perennials have become very popular in the trade; however, you should be familiar with each spe- cies and cultivar, as their cold tolerance varies. Growing Perennials in Georgia Figure 2. Georgia Plant Hardiness Zone Map. Zones are based on average minimum winter temperatures. Many While perennial plants are found in almost every nat- other factors, such as summer heat, summer night tem- ural habitat around the world, not all plants can grow peratures, humidity and rainfall, affect plant performance well when they are cultivated in a different region of the and adaptability. globe. The best plant performance is achieved when the landscape habitat closely matches the plant’s endemic environment. Until recently, most plant catalogs listed only the cold hardiness zone when describing how far north a plant can be expected to survive (Figure 2). Georgia has a diverse range of cold hardiness zones, ranging from zone 6a in the northeast corner of the state to zone 9a in the southern coastal part of the state. Plants growing in Georgia also face a challenging com- bination of high summer temperatures and high hu- midity (Figure 3). Academicians, nurserymen, growers, and landscape professionals have found that growing Figure 3. Average days above 900 F (30-year annual aver- perennial plants in the Southeast justifies using a heat ages rounded to next highest class) in the state of Georgia. tolerance map as well -- one that describes a plant’s adaptability to summer heat. The American Horticul- to reduced vigor, small foliage and weak, stunted plants tural Society (www.ahs.org) provides information on and. The effects of heat damage are subtler than those plant heat tolerance in a detailed interactive map. of extreme cold, which can kill a plant quickly. Heat damage can first appear in many different parts of the Heat tolerance is more difficult to evaluate than cold plant -- flower buds may wither, leaves may droop or tolerance; plants respond to cold by dying, to heat by become susceptible to pests, chlorophyll may disap- slowly declining. High night temperatures increase res- pear so that leaves appear white or brown, or roots piration; for every 160F rise in temperature, respiration may cease growing. Plant death from heat is slow and rate doubles. Plants use up their stored reserves, leading lingering. UGA Extension Bulletin 1424 2 Landscape Basics: Success with Herbaceous Perennials Georgia also has a very diverse ecology, with more than tion in sandy soils causes plant stress from drought. In 100 distinct environments and at least eight major habi- fact, more perennials die from wet winter soils than from tats. These are generally grouped within three geological cold in Georgia. Amend perennial beds with organic regions: Mountains / Northwest, Piedmont and Coastal matter to improve soil conditions. Plain. Figure 3 shows that the Coastal Plains experience more days of temperatures over 900 F compared to more Research has shown that plant growth in Georgia cli- northern regions (Mountains), which plays a major role mates is typified by earlier-flowering, taller plants with in plant performance across the state. weaker stems due to accumulated heat – more so than in more northern climates. Tall forms tend to collapse with- The last two factors to consider when planting peren- out support; therefore, dwarf selections are usually more nials are rainfall and soil. The entire state, including the effective. Fertilizer may not need to be applied as gener- Mountains, receives moderate to heavy rain, which var- ously, particularly for tall plants as additional growth is ies from 45 inches in central Georgia to approximately not desired. Leggy, lanky growth and fewer flowers result 75 inches around the northeastern part of the state. The if too much nitrogen is applied. Coastal Plains experience hot, humid summers with fre- quent afternoon thunderstorms and mild, drier winters. Soil pH and Mineral Nutrients Wet, cool springs are typical. Since perennials are expected to live in a certain area for many years, it is very important to be sure the soil is Rainfall, when considered with various soils, affects amended to fit the plant’s needs. In addition to organic perennial plant survival and adaptability to Georgia’s matter, pay attention to soil pH. Too high or too low soil climate. The soils in Georgia are varied; generally, the pH may affect nutrient uptake, stress the plant and re- Coastal Plains have sandy, well-drained soils that are duce the plant’s chances of surviving more than one year. poor in organic matter, while the Piedmont and Moun- tain regions have heavier clay or clay-loam soils that Most perennials prefer a pH between 5.2 and 6.7 (slightly retain water well, with organic matter present in some acidic). Soil pH affects nutrient uptake (Figure 5). This areas (Figure 4). “Goldilocks” range assures that two important micro- nutrients -- iron (Fe) and manganese (Mn) -- are being Drainage becomes a very important factor in plant sur- absorbed by the plant roots in optimal concentrations. vival. Inadequate drainage in clay soils creates favorable If the pH is below 5, both of these elements can interfere conditions for root rots, and insufficient moisture reten- with the uptake of other nutrients. If the pH is higher than 7, Fe and Mn can become deficient. Symptoms of nutrient deficiency include yellow (chlorotic) leaf tips and young foliage (Figure 6). Figure 4. Georgia soil map (for detailed information, visit www.dot.ga.gov). Colors indicate different soil types. Figure 5. Nutrient availability chart. The red box shows the pH range in which mineral nutrient uptake is optimal. Landscape Basics: Success with Herbaceous Perennials 3 UGA Extension Bulletin 1424 sium [N-P-K]) in a 3-1-2 or 4-1-2 ratio. A 12-4-8 fertilizer, for instance, is a 3-1-2 ratio, and a 16-4-8 fertilizer is a 4-1-2 ratio. Research shows that phosphorus, the middle number in the analysis, is held by soils and does not leach with rains or irrigation as much as nitrogen or potas- sium do, so it is usually needed in lower amounts. The most common types of fertilizer ap- plied to perennial beds in the spring are general-purpose 10-10-10 (N-P-K), 8-8-8 or 16-4-8.