Gardening: Right fertilizer makes the difference for acid-loving plants
When I did a walk-through of a friend’s garden, she mentioned some of her favorite shrubs were not looking too good in spite of the fact they were in healthy settings and getting enough water. The newer leaves on her rhododendrons and azaleas were taking on a lighter color and the veins in the leaves were beginning to stand out.
As we walked about, we found the same was true for some of the other shrubs and trees, including a river birch tree and several hydrangea paniculata.
The first clue to the problem was that all are acid-loving plants that can develop yellowing of leaves, a condition called chlorosis, when grown in alkaline soil. It’s caused when the element iron gets “locked up” in the soil and the plants can’t take up what they need. The soil test in this garden registered a pH of 7.8 and it’s irrigated with Detroit water that registers in the high 7’s. Acid lovers are happier in a pH of 4.5 to 6.5.
My friend was surprised at my diagnosis because she uses a popular water soluble fertilizer developed for use on acid-loving plants. Unfortunately these easy-to-use, fast-acting water soluble chemicals quickly wash through the soil and do nothing to change the pH of the soil. When overused, the corrosive salts left behind can burn the tender root hairs that take up nutrients and moisture while repelling earthworms and beneficial organisms that help enrich the soil.
Whenever I tour a garden that contains a display of exceptionally healthy trees and shrubs, I ask about cultural care. And in one of the best gardens I’ve seen, the caretaker told me they use a mix of Espoma Holly Tone and Milorganite, slow-release organic based fertilizers, mixed according to package directions. The combination is applied at the recommend rate in early spring — just at or before bud break, and at half strength in late fall after the leaves have fallen.
The granular combination is sprinkled evenly on the surface of the soil beginning 4 more inches from the base of the tree or shrub, out the end of the outer branch line and than covered with mulch. If the area is already mulched, rake it away, apply the fertilizer and replace it. Should you be unable to move the mulch cast the fertilizer mix on the surface of the mulch and scuffle it up.
Timely tip: Do your fall shopping for the garden now, before the Halloween and Thanksgiving rush.
Nancy Szerlag is a master gardener and Metro Detroit freelance writer. Her column appears Fridays in Homestyle. To ask her a question go to Yardener.com and click on Ask Nancy. You can also read her previous columns at detroitnews.com/homestyle.