Spring tree care – less is more when it comes to fertilizer

Trees are leafy havens for birds and daydreamers, and givers of fruit and shelter. We write poems about them and plant them as a sign of faith and hope in the future.

We love them. And we want to show it.

So we fertilize, spread chemicals at the first sign of a leaf spot, and perhaps throw in some Epsom salts, which can fix almost any plant problem, according to urban legend. And what could be more natural than adding kelp or phosphorus?

Sometimes, more is better for trees, but it almost always happens before or during planting. A soil test is the way to find out if a tree needs any supplements. A properly dug hole, cutting of circling roots that can strangle a growing tree, planting at the proper depth, and generous watering are keys to helping a tree survive its early days and later thrive.

Although May is the best time to fertilize trees in Minnesota if they need it, the reality is that most of the nutrients that trees need to thrive are already present in our soil. Adding unnecessary fertilizer can actually hurt.

That was the message from Linda Chalker-Scott, a Washington State University associate professor and extension horticulturist, when she spoke to tree-care professionals at the Minnesota Shade Tree Short Course earlier this year. Chalker-Scott is a stickler for using the results of research to guide horticultural practice. She is one of the founders of TheGardenProfessors Facebook page, which has nearly 20,000 followers.

"People love adding nutrients," she said. "But the research is starting to show more and more that it's not necessary."

Most of the time, you don't need to add anything. When trees fail to thrive, it's usually because they were planted too deep, are being strangled by girdling roots or haven't been watered during drought.

If an element is missing in Minnesota soils it tends to be iron, according to the U of M. Trees that are suffering from an iron deficit have chlorosis, where leaves turn yellow but the veins remain green. Trees that are growing in alkaline soils may have trouble taking up phosphorus. That's most common in western Minnesota, according to the U, but also can occur in areas where soil is very compacted, on new construction sites or where parking lots, building foundations and driveways were built. A soil test should reveal if there is a problem.

A liquid containing iron sprayed on leaves usually greens up the plant, but Chalker-Scott said it doesn't fix the underlying problem. In soils that contain too many nutrients — a condition that is common in the Twin Cities, where soil tends to have too much phosphorus — the heavy presence of one element may inhibit a plant's ability to take up another one.

"Fertilizing seems an easy fix," Chalker-Scott said. "But adding fertilizer without knowing what the situation is can just make things worse. ... People like to add bone meal and other stuff when planting, but that phosphorus just stays there and builds up. If there's too much phosphorus, roots can't take up iron and magnesium."

Sometimes young or newly planted trees and shrubs become chlorotic, but seem to outgrow the problem. Once plants are established, and their root systems spread and strengthen, they find it easier to absorb necessary elements, Chalker-Scott said.

Nitrogen is the best fertilizer for trees, according to Chalker-Scott. If trees are surrounded by lawn that's fertilized regularly, that usually is all the fertilizing trees need. If a tree that's been in the ground for a few years is showing extremely slow growth or signs of a nutrient deficiency, it may need fertilization. Here's the U of M's detailed information: https://bit.ly/2I3NGh1

Nitrogen is constantly being sucked up by plants, so it will never create a toxic soil, Chalker-Scott said. Her other recommendation is to mulch trees with arborist's wood chips. They conserve moisture around the base of trees and encourage development of beneficial fungi that will aid roots. Just keep the wood chips away from the base of the trunk to prevent fungi from affecting the bark, which can promote disease.

"If you have a new tree, I say be cheap and lazy," she said. "Don't spend on anything that doesn't do any good. Let nature do the work. Buy the tree, dig the hole to the proper depth, water and mulch well.

"If you really feel the need to add something, put a little bit of nitrogen on the soil before you put the mulch down. Trees do need nitrogen. They have to have that."