6 Gardening Myths Debunked

Your ideas about plants, pollinators and soil health may be outdated

Susan Moeller,

AARP

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En español

Published June 06, 2023

a gardener carrying a clay pot of flowers

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One of the charming things about gardening is the way home gardeners share information, often passing down tips and tricks that are decades old.

Just one problem: Many of them are wrong.

“A lot of these old-fashioned things are handed down from generation to generation,” says Robert Pavlis, an Ontario-based Master Gardener, blogger, educator and the author of several books, including two volumes of Garden Myths.

Just because you’ve always done something a certain way, doesn’t mean it’s actually what’s making a difference, he says.

There are so many variables in gardening, it’s hard to know if something really works without serious research and scientific methods.

Sometimes there’s a kernel of truth in old-time gardening practices. Marigolds supposedly ward off root-knot nematodes — microscopic pests that damage roots — from tomato plants, says Glen Bupp, a commercial horticulture educator with Penn State Extension in Pittsburgh. And there are certain varieties of marigolds that do have some efficacy against nematodes when the green plants are turned into the soil before tomatoes are planted, Bupp says.

“However, when they’re just alive, living as a normal plant in between plants, those compounds … aren’t being released into the soil,” he says. “So, it’s not doing its thing.”

So why do these customs persist?

Garden advice becomes twisted like a game of telephone, garden books go out of date and, of course, the internet and social media, while often helpful, can also contain misinformation. Online garden forums or sites sometimes spread pseudoscience or sell products in the guise of advice, experts say. One good way to truth-test gardening information: Check with your local extension service, an educational partnership between land-grant colleges (such as state universities) and the U.S. Department of Agriculture, or search on websites with .edu addresses that are affiliated with educational institutions.

“I think maybe 20, 30, 40 years ago, a lot of the university research was more focused on agriculture than on home gardening,” says C.L. Fornari, a Cape Cod-based gardener, lecturer, radio host and author of Coffee for Roses: ... and 70 Other Misleading Myths About Backyard Gardening. “With the knowledge that what home gardeners do in their yards and gardens makes a difference to our environment, there’s more research being done and also more publicizing of that research.”

Here are six gardening ideas you can abandon, according to experts:

1. Marigolds ward off garden pests.

Many people believe that marigolds help control mosquitoes or repel pests from tomatoes or other vegetables.

“The thing people forget is that insects don’t smell aromas the way we do,” Pavlis says. Just because a plant is fragrant to people, doesn’t mean insects will smell it or that it will produce enough fragrance to discourage pests, he says. And in some cases, it might make things worse.

“In fact, marigolds attract certain types of insects and you actually bring them to your garden,” he says.

2. Pots with holes need extra drainage help.

There’s no reason to put gravel or pot shards in the bottom of your pots for drainage, says Fornari. Professionals don’t do it and neither should you. A so-called drainage layer will trap your plant’s roots where there is neither water nor nutrition. Just fill the pot with planting medium and skip the pot shards.

“This is unnecessary, it’s a nuisance, and more to the point, it’s bad for plants. When it comes to drainage, that’s what the hole in the bottom of the pot is for,” she says.

3. Peony blooms need ants.

Ants are attracted to peony buds because the flowers emit sap that is higher in sugar than Coca-Cola, Fornari says. Whether your peonies bloom or not has nothing to do with the ants. “Nature would not necessarily design a flower that is so totally dependent on another creature for its existence,” she says. If your peonies aren’t blooming, you need to track down the real problem, such as a lack of sunlight.

4. Use your instincts to amend soil.

There’s all kinds of advice on what to add to your soil — Epsom salts for magnesium, coffee grounds for acid, lime for sweetness, sea salt for micronutrients, an entire encyclopedia of fertilizers for growth. But just stop until you get a soil test, all experts say.

For example, adding lime to your soil may actually be detrimental, says Bupp. “And year after year, basically you’re raising your pH and adding in huge amounts of calcium to your soil. So, you could be setting yourself up for actually some nutrient deficiencies,” he says.

Your extension service can do a reliable soil test and will have instructions on how to collect a proper sample. A test appropriate for home gardens is about $20 and can often be done by mail. You can also buy tests at local home and garden stores but they may be more expensive and the results less reliable than what you'’d get from an extension service.

5. Add compost to planting holes for strong roots.

One of the longest-standing traditions — and one of the hardest to kill off — is the idea of amending the native soil in a planting hole with compost, cow manure or peat moss, says Fornari. But university research has shown that’s the worst way to plant something, she says. And it’s contrary to nature, which just lets plants and seeds take root (or not) in native soil. By enriching the hole, “you create a five-star restaurant, and the plants’ roots don’t want to leave that to go to the fast-food joint next door,” Fornari says. “So you are creating reduced root systems.”

Instead, dig a wide bowl-shaped hole, fill it with native soil and then top-dress with a couple of inches of compost or composted manure.

6. Homemade remedies are best to ward off garden pests.

Don’t make the cure worse than the pest. Check any homemade recipes with your extension service; going off-label on some products might even be illegal, says Bupp. At the very least, it may hurt your plants. For example, dish soap is sometimes recommended to kill aphids. But it’s not the same as using a horticultural soap designed for the purpose, Bupp says. Dish soaps may contain detergents, which are too harsh for plants.

“The reason that these products do work on soft-bodied insects like aphids is because it removes their waxy cuticles and essentially dehydrates them. Well, you do the same thing to a plant,” he says. Also, household soaps may be sodium-based, he says. “One of the issues that you can see is an accumulation of sodium in the soils. It’s also much, much more difficult for a plant to process,” he says.

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Susan Moeller is a contributing writer who covers lifestyle, health, finance and human-interest topics. A former newspaper reporter and editor, she also writes features and essays for the Boston Globe Magazine and her local NPR station, among other outlets.