Companion planting: Why some plants are better with a buddy

Gin and tonic, Thelma and Louise, butter and popcorn — some things are just better together. The same is true in the garden. By emulating a natural ecosystem, companion planting (planting certain plants in proximity to other plants) can improve soil structure, add flavour to vegetables, make your garden less vulnerable to pests and disease and reduce the amount of time you spend pulling weeds. English cottage gardens have used the technique for centuries to reduce maintenance and to create a landscape that almost takes care of itself.

Both flowers and vegetables can benefit from having companion plants growing nearby so if you've been segregating your garden for the sake of colour or function it's time to yell 'snowball!' and shake it up.

To find out what we should be pairing with our plants and why, we consulted Jon Peter, curator and manager of Plant Records at Royal Botanical Gardens in Hamilton, Ontario.

Provide a B&B for pollinators

If you haven't yet heard how vital bees and butterflies are for the survival of civilization, one statistic will clarify it for you. It's estimated that one third of the food we eat is dependent on pollinators. So if you want to do your part for Mother Earth, it makes sense to give these workhorses a safe place to land.

Jon suggests attracting pollinators in every season by curating a diverse selection of companion plants that bloom throughout the seasons. Planting Geranium maculatum (spotted geranium) which bloom in the spring will help bumblebees and other early pollinators find pollen and nectar at a vulnerable time in their lifecycle.

In summer, try Asclepias syriaca (milkweed). Milkweed is a twofer in that it's not only a preferred food source for monarch butterflies, it's where they like to lay their eggs as well. Bees and butterflies also appreciate sources of high quality nectar so a patch of Penstemon digitalis (foxglove beardtongue) will lure them in. Symphyotrichum (asters) and Solidago (goldenrod) that bloom in late summer and fall are excellent sources of pollen and nectar and will give migrating and hibernating pollinators much needed energy.

For habitat, think about places where pollinators will overwinter. Plants such as Eutrochium purpureum (Joe Pye weed) or native grasses like Panicum virgatum (switch grass) have hollow stems that pollinators can lay their eggs in, so you'll want to leave them in the garden for the entire winter to allow insects to complete their development cycle.

Deter the villains, attract the heroes

Some companion plants not only deter pests, they can also attract "hero" insects that prey on the bad guys.

Like humans, some creatures are sensitive to smells, so John suggests planting Allium (onions) if you are trying to discourage aphids, weevils, borers and even moles. Allium sativum (garlic) not only deters insects, it can help fight off diseases such as black spot and powdery mildew, and Geranium (geraniums and cranesbills) repels pesky Japanese beetles, aphids and rose beetles. If ants are your problem, try a patch of mint close to whatever they're feeding on.

For your vegetable garden, Jon says Borago officinalis (borage) is an overall trusted companion plant because it deters tomato hornworms and cabbage moth larva, while attracting beneficial pollinators who will help your cucumbers and squash prosper. Umbel shaped flowers such as Coriandrum sativum (cilantro), Anethum graveolens (dill) and Petroselinum crispum (parsley) will attract those beneficial predatory insects, such as ladybugs, hoverflies and lacewings, that help prevent infestations.  

It's a trap!

Like a sacrificial lamb, trap crops are planted adjacent to the plants you're trying to protect, so that pests attack the trap instead. Jon recommends protecting your vegetables by planting Calendula officinalis (marigolds) as a lure trap for slugs and Tropaeolum (nasturtium) as a trap for aphids as well as a deterrent for whiteflies, cucumber beetles, bean beetles and potato beetles.

Soybean for the win

If you're spending too much time pulling weeds there's a companion plant for that too. Jon suggests Glycine max (soybean). Soybean not only acts as a mulch which discourages weeds by shading out any competition, it fixes nitrogen in the soil which gives the soil a higher nutrient content — and because weeds prefer a degraded, nutrient-poor soil, they're less likely to propagate.

Add flavour to veggies

Just like some friends help you shine a little brighter, some companion plants make vegetables taste better. For example, planting some Ocimum basilicum (basil) and Allium schoenoprasum (chives) very close to your tomatoes and carrots will improve their flavour and basil also repels mosquitoes and flies so adding it to your pots on the patio or deck will make your garden parties less buggy.