Science-based companion planting

Buy your own copy of Plant Partners
by Jessica Walliser at Amazon.

Much has been written about which plants work well together, and often the various human emotions, such as love and hate, accompany these descriptions. Extension agents, university professors, and other scientists had debunked most of the anecdotal benefits of traditional companion planting. 

But now there is new science on using various plants to:
- act as trap crops to lure pests away from crops,
- attract predatory insects that will reduce pest problems on crops,
- add diversity, which may confuse pest organisms.
- reduce weeds,
- increase pollinator populations,
- add nutrients, and more.

Jessica Walliser's new book, "Plant  Partners: Science-Based Companion Strategies for the Vegetable Garden" (published by Storey Publishing) is the winner of a 2021 The American Horticultural Society Book Award. Jessica is a rare two-time winner of this prestigious award and her book compiles this latest research on using plants to accomplish several benefits in vegetable gardens. 

It's also significant that Jeff Gillman, director of the USC Charlotte Botanical Gardens and author of "The Truth About Garden Remedies," "The Truth About Organic Gardening," and "Decoding Gardening Advice," wrote the Foreword for this book since he's spent quite a bit of his time debunking old gardener's tales of companion planting since there was no science to back it up. He ends the Foreword with this:

If you want garden dogma, then go somewhere else. This is a reference for the open-minded gardener who is willing to think through their gardening choices. It gives us the science-based information we need to make informed decisions when choosing which plants to place next to one another. Plant Partners offers more than just specific plant pairings, it encourages us to think about the relationships between plants, so that we can grow our best garden ever.

The eight chapters

1) The Power of Plant Partnerships--How does companion planting work?

2) Soil Preparation & Conditioning--From cover crops to living rototillers

3) Weed Management--Using living mulches and allelopathy to control weeds

4) Support & Structure--Plants that act as living trellises

5) Pest management--Luring, trapping, tricking, and deterring pest insects

6) Disease Management--Suppressing disease through plant partnerships and interplanting

7) Biological Control--Plant partners that attract and support pest-eating beneficial insects

8) Pollination--Bringing more pollinators to the garden through the perfect plant/pollinator matches

Trap crops are basically set up as more desirable alternatives to lure pests away from the garden and where the pests can be killed or removed from the area. For instance, blue hubbard squash attracts pickleworm adults, squash bugs, and other pests that damage squash family crops. To accomplish this, plant the blue hubbard squash several weeks before planting your main crop and when the pests arrive, kill them by using poison on the plant--either systemic or sprayed. An alternative is to pull the whole plant, bag it up, and throw out with the trash. You may need to keep a supply of new blue hubbard squashes to set in place again at the ends of the rows or around the edges of your squash crops throughout the season. 

Don't pick off this tomato hornworm!
The wasp larvae have eaten it from the inside.

Companion plants that attract predatory organisms enhance biological controls against various pests. An interesting example is sweet alyssum (Lobularia maritima), a commonly-used, low-growing garden plant in the cabbage/mustard family (Brassicaceae). The research shows that the adult hover flies or syrphid flies (Sphaerophoria spp.) are attracted to the alyssum flowers for food. Then their maggot-like larvae are voracious predators of aphids and other bugs. 

Also, the tiny parasitoid wasps are attracted to alyssum for the pollen and their larvae feed on hornworms, army worms and more. As gardeners we usually only notice them when their larvae pupate on the outside of the worms. They look like grains of rice stuck to the outside of the worms. Handpicking is a recommended practice for reducing hornworms on tomatoes or other crops, but when you see these larvae, don't remove that hornworm. Let the wasps complete their life cycles and that worm is a zombie by then and will not eat much before it dies. 

I talked with Jessica recently and asked her what else she wanted her readers to know. She said that the best science on companion planting has been done in agricultural settings, so she said she wants people to perform their own experiments with companion plants in their smaller vegetable operations to find out what works in their own gardens. 

One note for Florida gardeners: Because we grow our cool-weather crops through the winter, some of the pairings, such as planting marigolds with onions and cabbages, would not work well in North and Central Florida because the frosts would kill the marigolds during the winter. 

Reviews & a podcast

“Plant Partners offers more than just specific plant pairings; it encourages us to think about the relationships between plants, so that we can grow our best garden ever." — Jeff Gillman, PhD, Director of the UNC Charlotte Botanical Gardens and best-selling author of The Truth about Garden Remedies and Decoding Gardening Advice

"Finally, a science-based book that addresses the 'companion planting' concept of how and why plants can - and really do - play a tangible role in helping to create healthier, more productive vegetable gardens. Plant Partners is a fun and fascinating read. It is well-researched and Jessica Walliser’s ability to pack so much useful information into this book just adds to the experience. I enthusiastically recommend this book!" — Joe Lamp’l, executive producer and host of Growing a Greener World

Podcast of Margaret Roach's interview with Jessica Walliser, author of "Plant Partners: Science-based Companion Planting Strategies for the Vegetable Garden."  A way to garden podcast with Jessica Walliser

Intercropping in my own garden this spring

Here's an example of intercropping from my own 
garden this spring: meadow garlic, cabbage
(partly harvested), dill, and broccoli.

This 4' x 8' bed is next to the garage and has a permanent stand of meadow garlic (Allium canadense) at the west end. Last fall, I planted a square of cabbages (Brassica oleracea), a wide row of dill (Anethum graveolens), and a wide row of of broccoli (Brassica oleracea). What you may notice here is that the cabbage and broccoli are not just in the same family, but are different cultivars of the same species. They are two of the Cole Crops and ideally, I should have had more space separating them than just a row of dill, which is in the carrot family.  I'd also planted a number of different lettuces last fall, so in hindsight, I should have planted some of lettuces behind the dill. They would have been happy with the light shade offered by the dill later in the season. The broccoli should have been farther away. Next season, I'll plan better for my intercropping. 

I hope you'll read Jessica's book to improve your interplanting techniques and so much more. I love these new ideas for a more successful vegetable garden. 

Green Gardening Matters
Ginny Stibolt