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More than folklore: Placing certain plants and flowers together can reduce pests and weeds

Flowering multi-coloured viola plants bloom among lettuces and bok choy.
Lynn Beclu
Flowering multi-coloured viola plants bloom among lettuces and bok choy. Placing certain plants and flowers near each other in your gardens and raised beds can aid in nutrient-sharing as well as reduce pests and weeds.

Home gardeners have been placing certain flowers and vegetable plants together for decades. (I see you, marigolds near tomatoes!) Now, scientific research shows companion planting can be beneficial.

Companion planting — that's the practice of placing certain plants and flowers near each other in the hopes they complement growth and reduce weeds and pests — has long been in practice. Until recently, this planting technique lacked the scientific research that showed it was beneficial.

Now, more scientists around the country and globally have been doing tests on companion planting. Many of their findings can be found in a book that came out last year called Plant Partners by Jessica Walliser.

Science-based planting strategies

One companion-planting strategy that scientists studied has to do with plant placement for nutrient sharing.

Certain pairings that work well include plants that give off nitrogen in the soil as they grow and others that take it up.

Legumes like peas "fix" nitrogen, or turn it into a form other plants can use in the soil. That nitrogen can then benefit plants like lettuces, kale, Swiss chard and other greens that grow better with lots of nitrogen. These plants do well when placed close together in your gardens and raised beds.

Another studied way to successfully companion plant is to look at root depths of plants. Lettuces, for example, have very shallow roots. Carrots' roots grow very deep.

Because these two kinds of vegetable plants are mining different levels of the soil for needed nutrients, they won't be in competition with each other when you plant them near each other in your gardens.

Another plant-pairing method that home gardeners have used for decades has to do with controlling insects.

Certain vegetable plants give off scents that attract pests who take up residence, lay eggs that grow into larvae, then decimate the plant.

These plants are sometimes known as "plant traps." Research found that placing "plant trap" crops, like Hubbard squash, can attract squash vine borers — keeping them away from other squash plants. To avoid introducing those pests into your whole garden, try planting squash in a separate garden space altogether.

Other plant pairings that are scientifically tested use an aromatic method to throw off garden pests, like planting nasturtiums next to zucchini and summer squash.

The nasturtium flowers give off an aroma that masks the insects' favorite squash plants. If the garden pests can't find the squash plants, they are likely to lay fewer eggs, which means fewer larvae hatch and you minimize that risk.

This aroma-masking method works with pairing basil and tomato plants, too. Try planting basil — which puts off an aroma that confuses the tomato hornworm — and see if they lay fewer eggs.

A question about reviving blueberry bushes

Q: I was pruning dead branches from my blueberry bushes. I noticed one plant had black spots on the stems. They look like spider mite spots, but they're black and not white. What's up with my berries and how can I help the bushes? - Karen, in Elmore

A: We had a lot of rain this spring, so it could be a fungal disease on the leaves, so pruning off those dead branches is a good idea to clean up the plant.

You could also spray with neem oil, which is a fairly innocuous fungicide spray.

This is also a good time of year to fertilize your blueberries — especially if you need to lower the soil's pH level — and that could mitigate issues and make the plant stronger.

And then next year, maybe do a little more pruning of your blueberries so you can stimulate some new growth. That will invigorate the plant further and you'll have less dead branches and hopefully a bigger, bushier plant.

Asparagus: to fern or not to fern

Q: Two years ago, I emailed you about a woodchuck that was demolishing our asparagus and whether it would still be viable last summer. Well, good news, it was! But last year, I was unable to stay on top of the prodigious crop of the asparagus beetles and their hungry larvae. A second summer without good ferns. This spring, the asparagus are coming up but the stems are really skinny. Should I harvest the thin roots or let them go to fern from the get go? - Barry, in Ripton

A: The rule of thumb with asparagus spears is if they are narrower than the diameter of a pencil, you should let them go to fern.

Because you've had such damage to your asparagus from the asparagus beetles and the woodchuck, it may be best to just let them go.

I would also encourage you to fertilize your asparagus with compost now. Try putting compost all around the plant and make sure the soil has good drainage.

These tweaks will hopefully get your plant set up for a better season next year and hopefully with fewer asparagus beetles.

All Things Gardening is powered by you, our audience! Send us your toughest conundrums and join the fun. Email your question to or better yet, leave a voicemail with your gardening question so we can use your voice on the air! Call Vermont Public at 1-800-639-2192.

Listen to All Things Gardening Sunday mornings at 9:35 a.m., and subscribe to the podcast to listen any time.

Charlie Nardozzi is a nationally recognized garden writer, radio and TV show host, consultant, and speaker. Charlie is the host of All Things Gardening on Sunday mornings at 9:35 during Weekend Edition on Vermont Public. Charlie is a guest on Vermont Public's Vermont Edition during the growing season. He also offers garden tips on local television and is a frequent guest on national programs.
Mary Williams Engisch is a local host on All Things Considered.