GARDENING : Putting Your Experiments to the Test

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A good rule in gardening is never try anything new without a way to tell if it makes a difference.

For example, take Terry Mikel’s experiment this year with zinc sulfate to control floral thrips on his rose bushes.

“In my mind, I saw less thrip damage on the ones I used the zinc sulfate on,” says Mikel, a horticulturist with the University of Arizona-Cooperative Extension Service. “This wasn’t a double-blind research test, or anything like that. I have six rose bushes at home. I put zinc sulfate on three and left three without zinc sulfate.


“The only thing I was looking at was thrip control. In my mind, I saw less thrip damage on the ones I put the zinc sulfate on.”

In much of the West, gypsum (calcium sulfate) is touted by many professionals as a cure for many soil problems. Epsom salts (magnesium sulfate) are almost as popular in some areas. Adding organic matter to existing soil is often considered mandatory in planting new shrubs and trees.

Yet there are well-qualified experts who consider all or some of them worthless. It seems to depend on local conditions. The solution: make your own tests under controlled conditions.

“Unless you leave something undone or different, a check, there is no way to know the cause of whatever happened,” Mikel says. “This is pretty standard in research projects, but I think it is a concept that escapes most amateur gardeners and even a lot of commercial growers.

“If in your mind you can see the new treatment was better than the standard treatment or the check treatment, then you know it works. Otherwise it could just mean that the season was particularly good and all the plants would have performed as well anyway because of the weather, or whatever.”

He suggested such checking techniques should be routine on anything done differently, even simple procedures like applying a mulch. “How else can you know there is a reaction? If you’re going to try mulching a bed of, whatever, leave some undone. See if those that weren’t mulched need water more often. Or if different things happened to them,” he added.


“But unless you leave something undone, you will never be able to put your finger on the missing link. And you will never know for sure what worked.”

Elaborate checks aren’t required, he said, but keep records and probably pay the most attention to changes in fertilizers.

“Let’s say you’ve been using a certain fertilizer for 10 years in the garden and want to try something new. Use your regular fertilizer on a percentage of the plants and use the new fertilizer on the others. You know growing conditions are going to be the same. So the only missing link is what was changed.”