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Garden Help Desk: Using nitrogen fertilizer to ensure healthy fruit trees

By USU Extension - Special to the Daily Herald | Mar 2, 2024
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Most young fruit trees need at least a few years of growth before they're mature enough to bloom and set fruit.
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The Sensation box elder tree provides nice fall color in home landscapes.
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The male flowers on a Sensation box elder add color and beauty to the spring landscape. Pruning can be delayed until after this colorful display and may even reduce sap weeping from the pruning cuts.

I have a Santa Rosa plum that’s 3 years old and hasn’t ever flowered, but it puts on 3 to 4 feet of growth each year. The lower half-ish of the tree has a lot of pointy stubs that look like places branches have died or been bitten back. I do have deer issues, but I have not seen similar issues on my peach trees in the same area. Can you tell me what is going on and the proper treatment approach?

Too much nitrogen fertilizer can interfere with bloom on many flowering plant species, but in this case your tree is probably just still too young to bear fruit. Most plums begin to bloom and set fruit at about 3 to 6 years of age. You should be seeing blossoms in the next few years.

While you can’t do anything to hurry a young tree along to maturity, there are ways to make sure your fruit trees get the right amount of fertilizer each spring. (March is the ideal time to do this.) Here are two easy methods for deciding how much nitrogen a fruit tree needs.

Tree age. One useful rule of thumb is to apply 1 ounce actual nitrogen (N) per year of tree age, not to exceed 8 ounces per year. For example, if ammonium sulfate were used (21-0-0 or 21% N), you would need to apply five times the amount of ammonium sulfate to get the right amount of actual nitrogen (at 21% N, ammonium sulfate is 1/5 N). This means you would apply 25 ounces, or about a pound and a half of fertilizer, to a 5-year-old tree. Use this same calculation to find the right amount of total fertilizer, regardless of your fertilizer choice.

Annual shoot growth. Young non-bearing trees should produce 12 to 18 inches of new growth on branches each year. Most bearing trees should produce 8 to 12 inches of new growth per year. Peach trees are a little more vigorous. The new growth on fruit-bearing peach trees will be more at 12 to 15 or even 18 inches of new growth. If your trees are producing this amount of shoot growth, whatever nitrogen fertilizer they are getting is sufficient. If there is less new growth, the trees will benefit from additional nitrogen. If there’s more new growth, the trees would benefit from less nitrogen.

We just pruned a few branches off our 3-year-old Sensation boxelder tree and every cut is weeping sap. Will it be OK?

Your Sensation boxelder is going to be a nice addition to your landscape. I have one in my own yard and enjoy the spring and fall color. Your tree should be fine. Weeping like this is common with many tree species.

Maples (like your box elder), birches, elms, walnuts and honey locusts are just a few of the trees that are prone to weeping (sometimes called “bleeding”) when they are pruned in the spring. When they’re breaking dormancy in the spring, tree roots begin drawing moisture from the soil and moving sap up and throughout the tree to bring water and sugar to flower and leaf buds. This sap movement is more “enthusiastic” for some trees than for others, and some of that sap will make its way out the pruned ends of twigs and branches.

Trees aren’t the only plants that respond this way. You may also have seen grapevines weeping after they’ve been pruned in the spring.

This weeping doesn’t harm the trees and will eventually stop on its own. Resist the temptation to cover the weeping pruning cuts with paint, wax, tar, tape, etc. Coverings like these won’t stop the weeping, but they will increase the chances of disease or decay in the pruned branches. Let the tree manage the weeping on its own.

Keep in mind that shade trees need little if any pruning except to correct the occasional problem. If you do need to prune a tree that’s prone to weeping, pruning earlier (late winter) or later (very early summer) to help avoid the weeping. A little research about your own particular tree will tell you whether earlier or later pruning is best.


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