Fertilizing

Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent  

Poor fertilization practices can lead to problems with nitrates finding their way into surface water... streams, lakes, rivers... and into ground water. According to Mary Robson, WSU Area Extension Agent in the greater Seattle area, it’s important to apply fertilizers correctly to both provide needed nutrients to plants and to prevent runoff and leaching.

Robson directs her comments, not to the agricultural producer, but to home gardeners. She points out that at this time of year, many gardeners start thinking about applying fertilizer to their landscape and garden plants. Gardeners should apply fertilizers right before the plant buds break and start to grow. This time for our area would usually be in March and early April.

Trees, shrubs, and perennials plants utilize stored nutrient reserves during the first flush of spring growth. Fertilizer is applied to replace nutrients needed by plants and keep the plants healthy. Keep in mind that fertilizer is not plant “food.” Fertilizer simply provides the nutrients that a plant needs so that it can make its own food using energy from the sun by the process of photosynthesis

The following are tips that Robson offers gardeners regarding environmentally friendly fertilization of landscape and garden plants:

ESTABLISHED TREES & SHRUBS   Not all trees and shrubs will need fertilization. Large established plants will often get along fine with only a spring compost mulch or another organic mulch and no other supplemental fertilizer. What are the signs that a plant is growing well? If the plant develops good leaf color, puts on an average amount of new shoot growth and length per season, and generally appears healthy, fertilizer is probably not be needed.

YOUNG TREES & SHRUBS   A young landscape plant does need spring fertilization. If the garden has newly planted trees and shrubs, installed within the last two to four years, be sure to fertilize these to ensure the best growth possible.  Roots that are just getting established need extra nutrients in their second year.

WHEN TO FERTILIZE   It’s easy to over-fertilize, applying too many nutrients too often by assuming that landscape plants need lots of “plant food.”  Be sure to follow label directions for quantity and don’t add fertilizer to tree and shrub plantings after active spring growth has finished.  This means eliminating fertilizer applications to landscape plants after the middle of summer.  If you apply fertilizer too late in the season, the plant may fail to go into normal fall and winter dormancy and can be harmed by winter freezes.

NEWLY PLANTED TREES & SHRUBS   Robson recommends mulching newly planted trees and shrubs with two to three inches of mulch and then waiting to fertilize six months before applying any fertilizer to the newly installed plant.  This is good advice for the Seattle area.  However, many of our local soils contain so little nitrogen, that fertilization at planting time will greatly benefit plant growth.  Moderate amounts of slow-release fertilizer or tablets added to the planting hole can provide the needed nutrients as soon as the roots start to grow out of the original root ball.  If planting in a landscape or perennial bed, slow-release fertilizer and organic matter should usually be added to the soil when preparing the bed prior to planting.

Fertilizer bags often have confusing labels with different numbers.  Nitrogen is the main nutrient needed for good spring growth of woody plants.  Nitrogen is represented by the first number in the set of numbers on the fertilizer label.  Since that’s the primary nutrient needed by your trees and shrubs, you should look for a fertilizer where that number is larger than the other two numbers that represent the amounts of phosphorus and potassium.  Slow-release formulas are more expensive, but they’re best for your plants because nutrients are released gradually rather than all at one time.  Slow-release fertilizers generally lead to less waste and leaching of nitrogen and promote better plant growth.

If your trees and shrubs are situated in or next to the lawn area, they’re probably getting more than enough fertilizer through your regular lawn fertilization, especially if you aren’t using a slow-release material.  This can explain why you don’t usually need to fertilize these plants!  Yet, it’s important to note that the best times of year to fertilize lawns are not always the best time to fertilize trees and shrubs.  Washington State University Cooperative Extension recommends fertilizing lawns in September, early November, June, and May.

This dichotomy in fertilization times causes us a dilemma.  Should we fertilize the lawn at the right time or fertilize our trees and shrubs at the right time?  Fall fertilization can lead to problems especially when tender plants are fertilized late in the season.  Fall is the best time to fertilize our lawns to keep them healthy and thick enough to keep weeds out.  What should we do?  There may be no good solution.  However, try to avoid applying fall lawn fertilizers to areas above tender trees and shrub roots.  Slow-release fertilizers would help avoid a late flush of growth on woody plants that might be stimulated with a quick-release type of fertilizer.

How much fertilizer should be applied to individual trees and shrubs?  That’s one of those questions that’s hard to answer directly.  The amount of fertilizer needed depends on the size of the plant and the type of fertilizer applied.  To help you determine the amount of fertilizer you should apply to trees and shrubs, Washington State Unversity Cooperative Extension has a handy bulletin, “Fertilizing Landscape Trees and Shrubs EB1034," with the recommended rates and methods of application.  You can purchase one at your local county extension office.

How can you tell if your plants need fertilizer?  Look for plants that aren’t putting on good growth, where leaves are undersized and chlorotic or yellow.  Here again we have another dilemma, because these are also symptoms of other plant problems.  In many cases the symptoms of poor growth and yellow leaves turn out to be a root or soil moisture problem.  If the tree or shrub has trunk injury, root damage, girdling roots, root rot or excessive soil moisture or drought, the symptoms will be pretty much the same.  Fertilizer will not help these plants or solve the problem of poor growth.