Landscape Architect Articles

Fertilizing Ornamentals03-03-06 | News

Fertilizing Ornamentals

by Stephen Kelly, regional editor

A tulip bed at Pinehurst Resort. Soil for tulips should be quite loose (for good drainage) to avoid fungi and preferably alkaline, a mixture of sand and garden soil, not manure.. The application of fertilizer to bulb plantings is best done as a top dressing. To avoid ?EUR??,,????'??burning?EUR??,,????'??? a bulb or its roots, never mix fertilizer or fresh manure into the planting hole with the bulb. The high concentration of salts in some forms of nitrogen can dessicate plant tissue if it makes direct contact. The newly emerging roots on a bulb are very sensitive to these salts; death of the roots will result in bulb failure. The Netherlands Flower Bulb Information Center advises mixing slow release bulb fertilizer into the top few inches of soil once a year.

Editor?EUR??,,????'???s note: A good deal of information for this article was gleaned from ?EUR??,,????'??Care of Ornamental Plants in the Landscape?EUR??,,????'?? by Gary Wade, horticulturist, and Beverly Sparks, entomologist, both from the Cooperative Extension Service of the University of Georgia College of Agricultural and Environmental Sciences. Additional information came from the North Carolina Dept. of Agriculture and interviews with landscape managers Chris Burrows (Pinehurst Resort, N.C.) and Jim Kaufmann (National Gallery Sculpture Garden, Washington, D.C.).

Some plants (e.g., roses, English laurel) require more care and others are less worrisome (e.g., ornamental grasses, hollies, junipers). Azaleas, for example, need moist soil and shade from the glare of the mid-afternoon sun. Like most plants, azaleas also need good drainage. Improper care and choice of ornamental plants breeds weak flora whose lack of vigor makes them more susceptible to insect damage and disease.

Keep in mind low-maintenance landscape practices. It?EUR??,,????'???s certainly easier to care for plants native to a particular soil, terrain and climate than trying to nurse exotic plants. The low water needs of xeriscapes and employing integrated pest management (discriminate use of insecticides and encouraging friendly pests) are reaping not only attractive landscapes but saving supers time and money on T & O care.

Red bird azaleas (potted) and kumquats are enclosed within a Japanese boxwood hedge at the Nixon Library. Azaleas are surface feeders and need light, acidic soil. This is accomplished by mixing half the natural soil of the garden with an organic mixture: one-half peat moss, 30 percent soil, 20 percent sand and applying an acid-forming fertilizer. The boxwoods need well-drained soils and grow in soils ranging from slightly acid to slightly alkaline (pH 5.5 to 7.5). Based on the soil analysis, the proper amount of lime or sulfur and fertilizer can be added to provide proper nutrition. The kumquats are fertilized every 3-4 months (November, March, June, August) with a citrus fertilizer at a rate of 1-2 lbs. for every year of age. Fertilizer should be applied around the drip line of the tree, not next to the trunk.

The Soil Test

Chris Burrows, the superintendent for the Pinehurst Resort in North Carolina, and Jim Kaufmann, a horticulturist for the National Sculpture Garden in Washington, D.C., both rely on annual soil tests to guide them on fertilization and soil amendments. Mr. Kaufmann?EUR??,,????'???s property on the Washington Mall is known as the ?EUR??,,????'??clay swamp.?EUR??,,????'??

When the garden was installed about three feet of soil (clay) was removed and replaced with a custom soil blend?EUR??,,????'??+about 77 percent sand and the rest organic matter. Some of the ornamental beds have been completely dug up to put in new drainage and to add soil amendments (gravel, organic matter, bark mulch). Kaufmann employs IPM and also relies on extension service recommendations for herbicide use and what other horticulturists in his area find works best.

Table 1?EUR??,,????'??+Average nutrient content of various organic fertilizer sources. View full Size Image

Lime, Lime, My Kingdom for Lime

Yes, a silly parody of King Richard III?EUR??,,????'???s plea, but according to the North Carolina Dept. of Agriculture (NDCDA), lime contributes more benefits to soil than an other amendment. It neutralizes soil acidity, stimulates microbial activity, enhances the availability of nutrient elements and supplies essential calcium and magnesium and promotes plant growth.

Most bagged lime is a finely ground, high-quality agricultural grade dolomitic lime (contains calcium and magnesium carbonates). Lime recommendations on soil test reports are expressed in units of M, the same as lb./1,000 sq. ft. Lime, like fertilizer should be applied based on a current soil test. The suggested rate and application should raise the soil pH to 6.0-6.5, explains NDCDA, and keep it within the desired range for two to three years on sandy coastal plain soils and for three to four years on silt and/or clay piedmont and mountain soils.

For new gardens lime is broadcast over the surface and incorporated 4?EUR??,,????'???8 inches into the soil. For gardens and ornamental shrubs its broadcast then irrigated. The NCDA warns not to surface apply more than 50 pounds of lime per 1,000 sq. ft. at any given time. Overliming can reduce the availability of some micronutrients, a problem is common on sandy coastal plain soils. Lime residue will not harm plants and can be removed easily by irrigation or rainfall.

Oakleaf hydrangea (Hydrangea quercifolia ?EUR??,,????'??Pee Wee?EUR??,,????'???) border many of the walkways in the National Gallery Sculpture Garden. In the background pink flowered Mandevilla (Mandevilla splendins ?EUR??,,????'??Alice Dupont) climb the fa??? 1/4 ade of the Pavilion Caf????(C). Fertilizer for the annual planting beds and container plants is done biweekly, normally a 20-20-20 liquid.

Fertilization

Generally, most ornamentals benefit from 3-1-2 or 4-1-2 ratios of nitrogen, phosphorus and potassium (N-P-K). Phosphorus, the middle number in an analysis, does not leach with rains or irrigation as does N and P, thus the lower ratio requirement for P. Still, on ground where phosphorus has never been applied, a balanced ratio (8-8-8 or 10-10-10) is sometimes recommended.

Base your fertilizer choices on cost, types of plants, the soil type and the desired growth response. Liquid or water-soluble fertilizers are good choices for just planted annuals and herbaceous perennials as they need to quickly take in nutrients. Slow-release fertilizers benefit plants that store reserves in their roots, e.g., woody ornamentals. Some granular slow-release fertilizers last six to eight months after application. Nitrogen listed as ammoniacal has slow-release properties; nitrogen listed as derived from urea, urea-formaldehyde, isobutylenediurea, or sulfur-coated urea increases the release duration. Common slow-release fertilizers include Osmocote granules, Osmocote tablets, Jobe?EUR??,,????'???s Spikes, Once, Woodace briquettes, Agriform tablets and Milorganite. These fertilizers generally cost more per pound than general-purpose granular fertilizers (e.g., 10-10-10 or 12-4-8) but last longer.

You may prefer composting or organic fertilizers (bone meal, cottonseed meal, manures). Table 1 lists the average nutrient content of several organic fertilizer sources.

Table 2?EUR??,,????'??+Recommended fertilization rates for newly planted ornamentals during the first growing season. View full Size Image

When to Fertilize

Research published in the Journal of American Horticulture Science on nonbearing prune trees and their growth cycle revealed the tree begins actively absorbing nitrogen from the soil beginning in April, with the peak levels of nitrogen absorption between May and Sept. This is the most critical fertilization period. The rule of thumb is to apply fertilizer as soon as the plants begin to bud and stop fertilizing after the first fall frost.

Table 3?EUR??,,????'??+Recommended application rates of various general-purpose granular fertilizers on established ornamental plants in the landscape. View full Size Image

Frequency of Fertilization

One application of a slow-release fertilizer should suffice for the growing season, however, two or three applications are necessary for general-purpose granular (GPG) fertilizers, depending on the slow-release properties. For just-planted ground covers, annuals, herbaceous perennials and roses, light applications of GPGs (12-4-8 or 10-10-10) are recommended at four to six week intervals. During periods of little rainfall or drought or in nonirrigated areas, reduce the amount and frequency of fertilizer, as it can injure ornamental roots under drought stress.

Newly-planted ornamental trees and shrubs benefit from light applications of fertilizer made during the first growing season after transplanting (Table 2). Broadcast the fertilizer uniformly along the perimeter of the planting hole. Newly planted ornamentals are under stress and can be easily injured by overfertilization. Also, don?EUR??,,????'???t use weed-and-feed fertilizers in the vicinity as the herbicide may do damage.

Nitrogen controls plant growth, so application rates are based on this primary nutrient. Table 3 lists suggested application rates for several general-purpose fertilizers. The recommendation of fertilization rates is 3-5 applications on annuals, herbaceous perennials, roses and newly established ground covers; otherwise, low rates are recommended, particularly if you desire a lower maintenance landscape.

Jim Kaufmann (National Gallery Sculpture Garden) hedges a ring of little leaf lindens (Tillia cordata ?EUR??,,????'??Greenspire?EUR??,,????'???). The trees receive a soluble injectable fertilizer (PHZ) with mycorrhizal fungi, which has given good results.

Fertilizing Guidelines/ Precautions

The following are some do?EUR??,,????'???s and don?EUR??,,????'???ts prescribed by Gary Wade, horticulturist, and Beverly Sparks, entomologist, both from the Cooperative Extension Service of the University of Georgia College of Agricultural and Environmental Sciences, in ?EUR??,,????'??Care of Ornamental Plants in the Landscape.?EUR??,,????'??

Broadcast fertilizer evenly over the bed area. Make certain the foliage is dry when fertilizer is broadcast over the tops of ornamentals.

If fertilizer becomes lodged in the foliage of ornamentals, brush it off before irrigating.

It is not necessary to remove the mulch when fertilizing. Broadcast fertilizer on top of the mulch and water it in.

Trees growing in turf areas will obtain nutrients from the fertilizer that is applied to the turfgrass. Do not apply excess fertilizer to turf in an effort to feed trees; injury to the turf may occur.

When fertilizing trees, broadcast the fertilizer over an area extending two to three times the canopy spread if possible. Research has shown that tree roots grow far beyond the canopy spread on established trees.

Do not concentrate fertilizer in holes drilled under tree canopy. Research shows that broadcast application results in better growth.

Plants growing in shade generally require less fertilizer than those growing in the sun.

An effective technique for fertilizing annual flowers is to place a slow-release fertilizer such as Osmocote in the planting hole directly beneath the plant. This not only provides an even supply of nutrients to the plants, it also decrease the number of weeds when compared to broadcast application.