EB1034 FERTILIZING LANDSCAPE TREES AND SHRUBS

Transcription

1 EB1034 FERTILIZING LANDSCAPE TREES AND SHRUBS Woody ornamental plants require reasonable levels of fertility to thrive. High levels of fertility stimulate excessive and possibly undesirable growth. On the other hand, low mineral nutrient levels in the soil are likely to make plants grow poorly and lack vigor. Landscape plants stressed because of low fertility levels are also more susceptible to insect, disease, and other problems such as lack of hardiness. Plants that are producing smaller than normal leaves, light green or yellowish leaves (instead of normally dark green leaves) and shorter than normal annual shoot growth for the species, will benefit from fertilizer application if mineral deficiencies are the cause of poor growth. The presence of many dead twigs or branch tips also may indicate the need for feeding. However, these symptoms also may be caused by many environmental, insect, disease, and other cultural problems. It would be prudent to rule out any such causes before automatically embarking on a fertilization program. Having your soil tested by a reputable laboratory could be helpful in determining actual soil fertility levels. Fertilizers can be expensive and are manufactured using nonrenewable fossil fuels. Potentially they can enter water supplies if heavy leaching and erosion occur. It is important to fertilize woody plants only when needed, to apply reasonably precise amounts of fertilizers at the correct times, and to place fertilizers where they are available to the roots. Whether or not you carry out a regular yearly fertilizer maintenance program for landscape plants should depend on the inherent fertility of the soil and how well the plants are growing. Very sandy soils lacking sufficient clay and organic matter are quite prone to nutrient deficiency. Landscape plants in such soils often exhibit deficiency symptoms without a regular fertilizer maintenance program. Fertilizer application is likely to help newly planted trees and shrubs reach

2 their landscape potential. Fertilizing also benefits trees and shrubs which have undergone partial or complete defoliation by insects and diseases, or stress caused by digging or trenching in the root zone. Appropriate fertilizer application may stimulate recovery from winter injury. Soil tests can determine the levels of phosphorus and potassium in soils. Very low readings of these two minerals may indicate the need to add these nutrients to a fertility program. In most cases, potassium and phosphorus reserves tend to be sufficient for woody plants- only a straight nitrogen fertilizer will be required. Research indicates that nitrogen may be the only nutrient which will cause a growth response in woody landscape plants. The three numbers on any package of fertilizer always refer to the percent of nitrogen (N), phosphorus (P 0 5 ), and potassium or potash (K 0), always in that order. Many fertilizer formulations are available, but since woody plants generally respond only to nitrogen application, it would be appropriate to use formulations consisting only of nitrogen or ones in which nitrogen was the predominant mineral. Some examples are , , , (sulfate of ammonia), (ammonium nitrate), and (urea). Lawn fertilizers without weedkillers are acceptable for fertilizing woody landscape plants. FERTILIZER APPLICATION BY AREA Both deciduous and broadleaf evergreen shade trees, as well as needleleaf trees (conifers), can all be fertilized with to 6 pounds of actual nitrogen per 1000 square feet per year. Very poor soils or unthrifty plant growth would suggest using the higher amount. If the plants are growing well and the soil is known to be fairly fertile, use the lower end of the range. Flowering trees, particularly crabapple, and flowering shrubs probably should be treated with the lower figure, up to pounds of actual nitrogen per 1000 square feet per year. Higher amounts of nitrogen on these plants may stimulate too much shoot growth at the expense of flowers. Calculations to translate the actual amount of nutrient required to specific fertilizer formulations (such as 1-4-4) are not difficult. For example, to apply 3 pounds of actual nitrogen per 1,000 square feet by using a fertilizer, divide the desired amount of nitrogen (3 pounds per 1,000 square feet) by the percent of nitrogen in the formulation (1% or.1) or 3.1 = 14 pounds Fourteen pounds of would give 3 pounds of nitrogen to be used on the 1000 square feet of area, as shown in Table 1. Table 1. Approximate Amounts of Various Fertilizer Formulations in Order to Apply Either 3 or 6 lbs. Actual N Per 1,000 Square Feet. Type of Fertilizer* Amount of Fertilizer to Apply (ammonium sulfate) 3 lbs. N Per 1,000 Sq. Ft. 30 lbs. 5 lbs 16 lbs. 14 lbs. 6 lbs. N Per 1,000 Sq. Ft. 60 lbs. 50 lbs. 3 lbs. 8 lbs.

3 (ammonium nitrate) 14 lbs. 11/ lbs 9 lbs. 6 lbs. 8 lbs. 5 lbs. 18 lbs. 1 lbs (urea) *These are examples. Availability may vary from location to location. The area under a tree is usually not exactly 1000 square feet. To find the area in square feet where the fertilizer is to be applied, put four stakes in the ground to form a square which encloses the dripline of the tree or extends slightly beyond it (Fig. 1). Measure the distance between the two stakes along a side of the square and multiply this number by itself to get the area of the square. Divide the area under the tree by 1000, then multiply this number by the pounds of fertilizer needed per 1000 square feet. This number will give you the pounds of fertilizer needed for that particular tree. Example: If one side of the square is 0 feet long, then the area under the tree would be 0 x 0 or 400 square feet. Four hundred square feet divided by 1000 square feet equals 0.4. If 3 pounds of actual nitrogen were needed per 1000 square feet (from a fertilizer), then 14 pounds of (from calculations or from Table 1) times 0.4 for 400 square feet equals 5.6 or roughly 6 pounds of to be spread under the tree. FERTILIZER APPLICATION BY PLANT SIZE The amount of nutrients applied to woody ornamental plants also can be determined by trunk size, height or spread of the plant. For example, shade trees with a trunk diameter of less than 6 inches at 36 inches above ground should receive from.15 to.3 pound of nitrogen per inch diameter of trunk. Trees more than 6 inches in diameter can be treated to.3 to.6 pound of nitrogen per inch of trunk diameter. Flowering trees and larger shrubs can receive from.15 to.30 pound of nitrogen per inch of stem diameter. Shrubs, however, are often fertilized according to their height or spread. Use.05 to.10 pound of nitrogen per foot of height or spread. Figuring the amount of fertilizer to apply to an 8-inch diameter shade tree is not difficult. If.3 pound of actual nitrogen per inch of trunk diameter is desired, and assuming the same fertilizer formulation is used, multiply the pounds of N needed (.3) by the diameter of the trunk (8). Divide this number by the percent nitrogen expressed as a decimal (.1). In this case,.3 x 8 =.4 actual nitrogen.4.1 = 11.4 pounds of Round off to 11 or 1. HOW TO APPLY FERTILIZERS

4 Figure 1. Placement of Fertilizer Holes Fertilizers can be broadcast over the root areas of woody landscape plants and watered in immediately. If trees or shrubs planted in turf areas need feeding, it might be better to split the amount into several applications to avoid burning the grass. Make the first application as growth begins, the second application about 4 weeks later, and the third application, if necessary, to 4 weeks after the second application. If, however, the application is made during the winter when the turf is likely to be dormant, it may not be necessary to split the application. Trees and shrubs planted in regularly fertilized turf areas may not need supplemental feeding. If soil tests show phosphorus and potassium are deficient, apply these two minerals by use of a complete fertilizer. However, for them to be effective, place them in holes in the soil or inject in solution form. The feeder roots of woody plants (those which take up nutrients) are both inside and outside the dripline. The square area you used to calculate how much fertilizer to use is where the fertilizer should be spread, injected, or put into the holes. The holes can be made easily with a soil probe or a soil auger attached to an electric drill. When nitrogen is spread on the soil surface, it will quickly leach down into the root zone. To avoid competition from grass or other plants growing beneath trees, or possibly to avoid damaging them, place the fertilizer into holes. Holes 1 to inches in diameter are made about 8 inches deep (depending on plant size), and about feet apart (Figures 1, ). Putting the hole on an angle will give greater surface for the fertilizer to act. If holes are drilled or punched close to the trunk, large roots may be damaged. Divide the fertilizer evenly among the holes and water it in thoroughly. An approximate amount of fertilizer (from various formulations) is found in Table. A convenient way to do this is to put the desired number of tablespoonfuls of fertilizer (from Table ) into a plastic or paper cup and mark the level.

5 Then, just fill the cup to that level with fertilizer from the bag or a pail. Table. Approximate Amounts of Various Fertilizer Formulations to Add to Holes ( Feet Apart) in Order to Apply Either 3 or 6 lbs. of Nitrogen Per 1,000 Square Feet of Area. Amount of Fertilizer Per Hole Per 1000 Sq. Type of Fertilizer* Ft. 3 lbs. of Nitrogen Per 1,000 Sq. Ft. Ozs. Tbls (ammonium sulfate) (ammonium nitrate) (urea) to 4 3 to 4 1 to 11/ 1 *These are examples. Availability may vary from location to location. WHEN TO APPLY FERTILIZER Although there is some controversy over the best time to fertilize woody plants, recent research indicates, at least as far as deciduous plants are concerned, that after the plants have started to grow in spring and have acquired some leaf surface, may be the most effective and efficient time to fertilize. Late fall, late winter, and early spring are also times when fertilizers can be applied, but some loss of nitrogen may occur through leaching and denitrification. Fertilizer, with the exception of slow-release types and phosphorus, should not be put into planting holes when planting trees and shrubs. There is always the possibility of burning roots, particularly if fertilizer is overapplied. Nitrogen fertilizers can be applied at a rate of.05 to. pound (1-3 ounces or 3-7 tablespoonfuls) actual fertilizer (the way it comes in the bag) to a 3- x 3-foot area on the soil surface around each plant after planting. Fertilizing landscape plants after mid-july with high nitrogen fertilizers is not recommended in most Washington areas. It may stimulate growth which will not have time to harden off before fall and winter frosts. Wait until after the plants have become dormant in the fall or until they initiate growth in the spring. However, fertilizing flowering shrubs (deciduous and evergreen) in mid-july with a fertilizer containing only phosphorus and potassium (such as ) may increase flowering and hardiness the next season. POINTS TO REMEMBER Never (positively never-don't even think of it!) put any type of fertilizer containing a herbicide into holes around

6 shade trees or large shrubs. Do not use a fertilizer containing a herbicide unless those plants are specifically listed on the label of the product. Herbicides may have specific toxicities for desirable plants, too. Do not use any lawn fertilizer containing,4-d, dicamba, or MCPP formulations under shade trees or other ornamental plants. Fertilizer applications are not good without moisture. If conditions are dry, irrigation will be necessary. Do not apply dry fertilizers to wet leaves of turf, shrubs, or trees. Fertilizers containing water-insoluble, organic nitrogen sources may take 3 to 8 weeks to break down to a usable form. Time the applications accordingly. By Ray Maleike, Ph.D., Washington State University Cooperative Extension horticulturist, WSU Puyallup, and George Pinyuh, former WSU Cooperative Extension King and Pierce counties area horticulture agent, Seattle. College of Agriculture and Home Economics, Pullman, Washington Issued by Washington State University Cooperative Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, Cooperative Extension programs and policies comply with federal and state laws and regulations on nondiscrimination regarding race, color, national origin, religion, gender, age, disability, and sexual orientation. Trade names have been used to simplify information; no endorsement is intended. Evidence of noncompliance may be reported through your local Cooperative Extension office. Slightly revised October Reprinted August Subject Codes 55, 374. A EB1034 TOP