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David Robson: Fertilize trees to help root growth

David Robson

It seems weird that November is the ideal time to fertilize most trees and shrubs.

First, you’re probably looking at the leaves littering the ground and thinking why would you want to have the tree grow more and produce more leaves that you’ll have to rake, bag, compost or make magically disappear?

Second, leaves are dropping. It’s not as if the plant is growing much, especially as temperatures start dropping. How will the fertilizer really help the plant?

Lastly, it’s going to snow sooner or later.

Fertilizers work best when the bacteria in the soil can help convert it to forms that roots absorb.

There’s no doubt fertilizing will make more leaves. Nitrogen means green shoots and green leaves. And those green leaves turn yellow, orange and red. Come October and November, you find them on the ground.

You can eliminate the nitrogen and go for the phosphorus to encourage roots, but you’ll also promote more flowering and fruiting, which might not be bad unless you have a sweet gum or bur oak tree. The flowering part might be good for many spring blooming trees such as dogwoods, crabapples and redbuds. Technically, though, you don’t need to fertilize redbuds, as they can fix their own nitrogen like peas and beans.

What if you forgo nitrogen and phosphorus and concentrate on potassium? It’s a possibility if you want to improve rooting and the overall vigor of the plant, but how do you tell if you succeed? Vigor depends not just on nutrients but also on rainfall, temperatures and soil quality.

Sure, you get more leaves, but you get stronger limbs and branches. You end up with a taller plant that will provide more shade, which can help keep down summer’s energy bills. Deciduous trees that shed their leaves, allowing in winter’s sunlight, can help you reduce your heating bill, though you could argue that your heating bill would be reduced even if you didn’t have a tree.

Applying fertilizer in the fall may not appear to benefit the trees, but it works like winterizer fertilizers work for your lawn.

As long as the ground isn’t frozen, roots continue to grow. For most shade trees and flowering shrubs, they thrive when soil temperatures are between freezing and 65 degrees. When soil temperatures are higher than 65, root growth slows.

Because roots are between 8 and 18 inches deep, they continue to function until the ground starts freezing. If we get sufficient snowfall, the ground may freeze only a couple of inches deep. Without snow cover, the frost level depends on how cold it gets and how long it stays cold.

As soon as the ground starts to thaw, roots start to grow.

When roots grow, they absorb water and nutrients. Because they can’t use the nutrients when the plant is still dormant, roots store nitrogen, phosphorus and potassium in forms that can be used as soon as the plant breaks buds in the spring.

The advantage is that the nutrients are already in the plant and available come spring.

Fertilizing isn’t difficult. Just remember that getting the nutrients to the roots is the important thing.

When you fertilize your lawn, some elements eventually leach down to the tree’s roots — though grass roots, if the turf is thick, are likely to grab most of the nutrients.

You can fertilize a little more in the fall, but you have to temper the excess with the potential for burning the grass. If you fertilized the first week of October, you won. All the rain moved the nutrients past the grass root zone and into the tree’s roots.

If you did that, play the lottery. You’ll probably win.

Another method is to punch holes in the ground with a 24-inch long metal rod about ¾ of an inch in diameter. Drive the rod into the ground at the tree’s dripline or outer branches 18 inches deep. Pull it out, and add six ounces of garden fertilizer. Move around the dripline 3 feet and make another hole. Repeat.

Once you’ve encircled the tree, move out three feet and create another ring of holes filled with fertilizer.

If the tree’s diameter is greater than 20 feet, make another ring three feet moving toward the trunk.

You don’t need to fertilize every year. Every five to seven years is enough.

And you don’t need to fertilize if the tree is producing 18 to 36 inches of growth each year. That’s sufficient for just about all plants except some dwarf specimens. Too much growth, and the wood is soft and brittle, likely to break in any location that suffers ice, snow and wind storms.

Besides, not fertilizing can save money, time and ultimately the groundwater supply.

David Robson is a horticulture educator for the University of Illinois Extension. For more gardening information or for your local extension unit office, go towww.extension.uiuc.edu/mg.