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Caring for Trees

Tree Fertilizer, Part 2: When to Fertilize Trees

By Keith BrownPublished Updated

Timing might be the most important factor to successful tree fertilizer applications. Even if you use quality products you can injure a tree by providing the wrong fertilizer at the wrong time of year. I like to describe tree fertilizing by comparing to grass fertilization practices. You can fertilize grass virtually any time of the year with a variety of products and get the same results: greener grass and faster growth. This is because grass is in a constant state of growth (evident by weekly mowing needs). Trees are not in a constant state of growth, so you can’t apply a product at any time. Instead, trees go through a series of growth stages throughout the year.

Most of a tree’s annual root growth takes place in the fall. So, the fall is the best time of year to apply a conventional fertilizer. A conventional fertilizer consists of nitrogen-phosphorus-potassium. This is the kind of treatment that is going to directly stimulate growth. If you are going to directly stimulate growth for a tree it should be targeted toward the roots. There are other products you’ll need in your arsenal for a really great treatment regimen; see my Tree Treatment Products article for more information on this.

In the early spring trees are focusing most of their energy on foliage development. All of our native hardwoods produce an entire new set of foliage every spring. If you apply a conventional fertilizer when the foliage is growing you are going to wind up with abnormally large foliage. This can be a big problem if there isn’t a root system to support those oversized leaves. When our hot summer days get here you can get major heat stress and foliage burn from over sized foliage. Applying some humic acid and/or growth hormones during this time of year will help develop thicker, greener leaves. This is the direction you need to go. Thicker greener leaves will be better and conserving water when heat stress gets here and they will be better energy producers all year long.

Throughout most of the rest of the growing season trees in Austin aren’t growing much at all. This is primarily an energy producing time. The trees are getting ready for the fall’s root growing season and storing energy for next spring’s foliage development. Applying a fertilizer in the summer will likely not produce anything but fertilizer leaching into the adjacent properties and ground water supply. However, there are some specialized fungi you can apply in spore form to the soil that can help. They are called mycorrhizae (my-core-rize-ah). These fungi attach to the root system of the tree; they grow their own root like system; then, share what they absorb with the tree. This is a great way to improve the water absorbing ability of the tree during the hottest and driest time of the year. Another good thing you can do in the spring is apply humic acid to the soil. This stuff is the by-product of natural composting and is readily available to buy at most garden stores. It is usually sold as a “root stimulator” or “compost tea”. Look at the label of these products and you’ll see they contain a significant percentage of something that begins with “hum”.

And, don’t forget about diagnosis. You need to know the state of health of your trees before you can expect to keep them healthy. Look at my articles about sick trees.

The tree fertilizer series

  1. Tree Fertilizer Part 1. An Introduction.
  2. Tree Fertilizer, Part 2: When to Fertilize Trees — this post
  3. Part 3. How to Apply Treatments to Trees
  4. Part 4. Tree Fertilizer Products. What to Use.

Common questions

When is the best time of year to fertilize a tree in Austin?

Most of a tree's annual root growth takes place in the fall, so the fall is the best time of year to apply a conventional fertilizer. That is the kind of treatment that is going to directly stimulate growth, and if you are going to directly stimulate growth for a tree it should be targeted toward the roots.

What should I apply to a tree in early spring?

In the early spring trees are focusing most of their energy on foliage development — all of our native hardwoods produce an entire new set of foliage every spring. Treatments that help develop thicker, greener leaves are the direction you need to go. Thicker, greener leaves will be better energy producers all year long.

Should I fertilize my trees in the summer?

Through most of the rest of the growing season trees in Austin aren't growing much at all. This is primarily an energy producing time — the trees are getting ready for the fall's root growing season and storing energy for next spring's foliage development. Applying a fertilizer in the summer will likely not produce anything but fertilizer leaching into the adjacent properties and ground water supply. There are, however, some specialized fungi called mycorrhizae that you can apply in spore form to the soil and that can help. They attach to the root system of the tree, grow their own root-like system, and share what they absorb with the tree — a great way to improve the water absorbing ability of the tree during the hottest and driest time of the year.

Can you hurt a tree by fertilizing it at the wrong time of year?

Yes. Even if you use quality products you can injure a tree by providing the wrong fertilizer at the wrong time of year. If you apply a conventional fertilizer when the foliage is growing you are going to wind up with abnormally large foliage. That can be a big problem if there isn't a root system to support those oversized leaves, and when our hot summer days get here you can get major heat stress and foliage burn from over sized foliage.

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