Meeting the fertilizer needs of trees and shrubs doesn't have to be a mystery. Just like people, plants have basic nutritional needs. And just like people, they need more of some nutrients than they do of others. Plant nutrients can be grouped into macro-nutrients (those they need a lot of) and micro-nutrients (those they need in small amounts). Every package of fertilizer should give its nutritional value. Usually it is indicated by three numbers such as 10-20-10. Those numbers represent the macro-nutrients nitrogen, phosphorous and potassium. 10-20-10 means that package contains 10% nitrogen, 20% phosphorous and 10% potassium by weight. Most fertilizers also contain some of the micro-nutrients. They may be specifically identified or the label may just indicate that they are included. Fortunately, almost all the micro-nutrients plants need are already available in the soil.

When you walk over to the fertilizer section of the garden center you will notice there are lots and lots of choices, some of them with the same nutritional analysis. How do you decide which one to use? If you can, take a few minutes to learn a little about synthetic vs. organic fertilizer and water soluble vs. dry formulas. They are all good products that will meet the needs of your plants, but you also want the fertilizer you choose to fit your needs. Some are easier to use than others; some are less expensive and some last longer than others. For more information, check out Bachman's Information Sheet Fertilizing Basics.

Do all trees and shrubs need to be fertilized? Trees and shrubs that are young and actively growing will perform much better if they are fertilized. On the other hand, they won't die if they have to rely on their own resources. Research as to whether or not mature plants should be fertilized is less conclusive. We do know for sure that mature trees need a lot less fertilizer because they are growing much more slowly.

Are there any circumstances in which trees and shrubs should not be fertilized? It is best to avoid fertilizing when trees and shrubs are newly planted (their first growing season) and when they are not healthy (unless a trained arborist or horticulturist has looked at the tree and diagnosed a specific nutritional deficiency).

Don't the trees and shrubs get enough fertilizer from what is put down on the lawn? Actually, they do make use of some of that fertilizer. That can be both a good thing and a bad thing. Too often trees and shrubs are damaged when they absorb lawn fertilizers that contain herbicides. When you are growing flowering trees or shrubs, lawn fertilizers often supply too much nitrogen and not enough phosphorus. The result may be lilacs or crabapples that produce lots of luscious green growth and not many blooms. Until a tree is relatively mature with an extensive, far reaching root system, it is best not to rely on your lawn fertilizers to provide all their nutritional needs.

Why do trees and shrubs have special fertilizing needs when plants in nature get along without it? In nature, plants rely on sunlight, rain and the nutrients in the soil. Nature limits the types and numbers of plants by the relative amounts of these basics available. In our landscapes, we grow many plants together with differing requirements, often in a variety of soil types, surrounded by competing grass. If they are to thrive, we have to supplement what Nature can provide.

What makes fertilizing trees and shrubs any different than fertilizing any other plants? In our climate, all plants are either herbaceous or woody. Woody plants have parts (such as trunks, branches, twigs, or evergreen needles or leaves) that live over from season to season. That gives them a big head start each year. Herbaceous plants do not have that advantage. They start from the soil line in spring and die back to the ground in fall. That means they have to expend a tremendous amount of energy during the growing season to produce all those stems and leaves and to help them out, we need to make sure they have the fertilizer they need.

Is there a difference between the fertilizer needs of woody plants and other plants? Most importantly, woody plants shouldn't be fertilized late in the growing season. Since the natural response to fertilizing is growth, it is important that any new growth has plenty of time to harden so it will survive our severe winters. To avoid this problem, we do not recommend fertilizing woody plants after August 15.

Do all trees and shrubs have the same fertilizer needs? Most trees and shrubs are grown for their foliage and structure but there are also some varieties grown for their beautiful flowers. Plants that are grown especially for their flowers need extra phosphorus. This is the middle number in the analysis (10-20-10). Phosphorus encourages blooming as well as strong roots and disease resistance. Trees and shrubs not grown for their flowers need less phosphorus and more nitrogen. Nitrogen is the first number in the analysis (10-20-10) and encourages leaf growth.

Are there different types of fertilizer for woodies? Fertilizers come in several forms. Fertilizers such as granulated 10-10-10 are loose, dry products; the same fertilizer is often compressed to form stakes or tablets to be driven into the ground. There are also liquid forms that can be injected into the soil or poured on. Most of the feeder roots capable of taking up fertilizers are within the top 12-18 inches of the soil surface. Roots close to the trunk are heavy conduits for the finer roots located at or beyond the dripline where the rain falls. They absorb very little fertilizer or water. On average, feeder roots extend out as far from the trunk as the tree is tall. Apply fertilizer in a ring around the tree starting just inside the dripline and extending several feet out beyond the dripline.

How do I know what kind to use?
There isn't just one right answer. The fertilizer must match both your needs and the needs of the plant. Dry fertilizer is inexpensive. If you are spreading it on the surface, dry fertilizers are easy to apply. If you are drilling it in, it will take more of your time and labor. Spikes are quick and easy to use, but they are a little more expensive and their nutrients aren't as evenly spread around the tree. Root feeders are easy, but it takes time to move them from site to site for good coverage and their nutrition doesn't last as long.

What would be a good dry fertilizer for trees and shrubs?
A quality product such as Bachman's Garden Food 10-20-10 works well with flowering trees and shrubs. With non-flowering woodies, a basic 10-10-10 works well.

How should dry fertilizers be applied? You have two options. One is to spread with a lawn fertilizer spreader but you have to be careful not to over-fertilize any surrounding plants. The better option is to drill or punch holes into the ground about 2-3 feet apart and 6-8 inches deep on a grid system covering the area to be fertilized. Divide the total amount of fertilizer needed among the holes. Cover each hole with a handful of soil.

Is there a good organic alternative for trees and shrubs? Both Sustaneâ and Renaissanceâ are great balanced organic fertilizers that work with trees and shrubs. Organic fertilizers are naturally slow-release.

Can I use a product such as Miracle-Gro? Water soluble fertilizer such as Miracle-Gro or Excel Gro™ can be used as a supplement the first few seasons, but isn't very effective with trees and shrubs once they are well established. It washes through the soil too quickly.

Are the root feeders a good idea? To be effective, you need to understand a few things about root feeders. First, it is essential that they are not pushed too far into the ground. In fact, six inches is ideal. Because the probe is 2-3 feet long, it is tempting to push it further into the ground. But all you want to do is push it far enough to bypass the surface root systems of the surrounding turf or other ornamental plants. Research has shown that no matter how big that plant is, almost all of the roots that actually absorb water are within the top 12 inches of the soil. Trees and shrubs have roots that go deeper, but they are thicker roots whose job it is to stabilize the plants, not absorb water and nutrients. It is also important to remember that the fertilizers used in root feeders are water soluble and are used up quickly. Nutritionally, they aren't much different than using a product such as Excel Gro™ Miracle-Gro.

Feeder root systems develop slowly extending into the surrounding soil as the plant grows. Be sure to apply the fertilizer to the feeder roots. Be sure to follow directions on the fertilizer package for application rates. Tender young roots are easily burned if the fertilizer is applied too heavily.

What about slow-release fertilizers? Organic fertilizers are naturally slow release. Products such as Osmocoteâ are not well suited for trees and shrubs because they would still be fertilizing late in the growing season, which isn't advisable in our climate. There are slow-release fertilizers professional arborists can inject into the soil that do work well.

How often should woody plants be fertilized? During the first year after they are planted, they should just be watered or given a very weak solution of fertilizer, such as Start-Upâ. Once they are established, trees and shrubs profit from being fertilized yearly.

What time of year should trees and shrubs be fertilized? In cold northern climates they can be fertilized either in the spring as soon as they show signs of new growth or in fall as soon as they have gone dormant. During both of these periods, the roots are actively growing. Avoid fertilizing during the last 6 weeks of the growing season.

Are there any trees and shrubs that don't like to be fertilized? There are varieties of trees and shrubs that would do well with what Nature provides if they were growing in their natural settings. But when we plant them in our landscapes where they are stressed by compacted soil and competition from grass, fertilizers not only help them grow but keep them stronger.

How will I know if the plants are getting enough fertilizer or the right fertilizer? If your plants aren't growing or blooming as much as you think they should, they may need a boost of fertilizer. With some plants, the leaves will be a lighter green than normal if they need fertilizer. But several other factors can influence plant growth or color, so it is a good idea to bring a sample in and ask a horticulturist before assuming fertilizer is the answer.

Is there a danger of over-fertilizing? Too much at one time can be hard on any plant, especially if the soil is dry. Follow the directions on the package for application rates and that shouldn't be a problem. With all plants, fertilizing too often may result in excessive growth that is weak and susceptible to problems. Over-fertilized plants are more easily stressed by lack of water, excess water, insects or diseases. For trees and shrubs, too much fertilizing may weaken their root system and make them less winter-hardy.

Are there any other tips for fertilizing trees and shrubs? Just be sure to follow all the directions on the package and try to spread the fertilizer out evenly. In dry weather, water the plants the day before fertilizing.

Also, water in dry fertilizer after it is applied. This helps activate it right away and keeps it in place. Keep in mind that fertilizers will leach through sandy soils more quickly than heavy, clay soils.

Additional Bachman's Information
Fertilizer Basics
Fertilizing Annuals and Perennials
Understanding Your Soil
Earth Friendly Solutions

Recommended Products
Sustane and Renaissance fertilizer
Bachman's Garden Food 10-20-10
Ross Root Feeders and Fertilizer Cartridges
Jobe's Tree Spikes
Excel Gro™ Water Soluble Fertilizer


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