Companion Planting in Vegetable Gardens

I've been reading and learning a lot on this topic recently, and chose to summarize what I'm learning in a document that I decided to post here, for ease of future reference.  I'm unsure about how the formatting will transfer, but we'll find out.

Note:  I have also looked at the book Carrots Love Tomatoes by Louise Riotte recently. I believe that this author may have stumbled onto the fact that some plant combinations work better than others, but I find that the book that I’ve chosen to reference here is easier to follow for people whose brains work like mine.  That is, after a principle has been established by referencing formal research, specific applications of conclusions arising from that research are suggested. 

For my own planning purposes, I hope to focus on 16 different garden crops (the kinds that are part of my “survivor” collection of open-pollinated seeds), identifying companion planting remedies (flowers as well as other vegetables) for problems that I’ve observed in my own garden. I may be able to do this in the form of a chart to add to my collection of garden charts. 

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Reference:  Plant Partners:  Science-based Companion Planting Strategies for the Vegetable Gardener by Jessica Walliser, Storey Publishing, 2020. All information below and many quotes are taken from this book.

Definition of Companion Planting:  The close pairing of two or more plant species for the purpose of enhancing growth and production or trapping or deterring pests. 

Main Idea

Plants have interactions with each other, with the environment, and with the life forms around them, especially insects.  One of the best ways in ensure that these interactions are beneficial for a desired crop is to increase the diversity of the plants around the desired crop.  Although research has uncovered some specific reasons for why certain crops seem to thrive in the presence of certain other plants, much is still unknown about how plants interact with their neighbors.  For this reason, keeping an open mind, making careful observations, and being willing to take small steps in the right direction seems wise.  Seeing the garden as an ecological habitat is a better mindset than to see it only as a canvas on which to impose the will of the gardener in hopes of maximizing yields for human consumption. 

Mechanisms at work in Companion Planting (Diversity = Stability)

1.  Chemical messaging—Plants and insects can release semiochemicals into the air that have the effect of calling in predators to eat a pest (HIPVs—herbivore-induced plant volatiles), warning neighboring plants to “arm” themselves, spurring faster growth, backing off from their neighbors (plant shyness, observable when canopies “refuse” to overlap).  This occurs above-ground.

2.  Fungal association—Below-ground mycorrhizae (various species of fungi) that colonize on roots of living plants extend their reach through the soil (via their “roots”—hyphae) and transfer along their length nutrients and moisture, making them available to adjacent roots.  Fungi also seem to be able to send communication signals to accomplish many of the same functions as the above-ground semiochemicals. 

3.  Resource competition—When desirable crops out-compete undesirable ones for resources, we consider the competition a benefit.   At other times, companion plants will share nutrients and access to water with their neighbors, so this is a benefit in the absence of competition. 

4.  Plant diversity/allelopathy—While seeming to be at odds with each other, these two mechanisms actually can end up as allies.  Allelochemicals are substances used to compete with other plants or fungi (i.e. neighbors), limiting their growth.  Used positively, allelochemicals can be used to limit weed growth around desirable plants.  Also, since some plants are not affected by allelochemicals, they can thrive next to plants producing them, while other competition is held at bay.  Sorting out suitable plant partners for desirable crops often has the effect of increasing plant diversity in the garden, making the whole garden more stable, resilient, and productive. 

5.  Nutrient absorption—One plant can improve the ability of another plant to absorb nutrients by altering one or more of these factors:  a.  Soil structure  b.  The concentration of nutrients present in the soil  c.  How strongly the nutrients are bound to the soil  d. How mobile that particular nutrient is within the soil.

Beneficial Results of Companion Planting

1.  Reduced pest pressure.  Utilizes luring, trapping, tricking, and deterring pests to minimize damage.

2.  Reduced weed pressure.  Utilizes living mulches through crowding or shading.  Allellopathy (growth inhibition produced by the roots of some plants) may also be used. 

3.  Reduced disease pressure.  Mechanisms:  a.  Generating antifungal compounds or conditions in the soil  b.  Supporting beneficial microorganisms  c.  Reducing the splash-up effect  d.  Limiting certain insect-transmitted diseases  e.  Improving the overall health of the soil and the plants  f.  Preventing a buildup of pathogens in the soil 

4.  Improved soil fertility or structure.  Mechanisms:  a.  Using plant roots to break up heavy or compacted soil  b.  Using living plants which exude beneficial chemicals through their roots, thus feeding soil microbes  c.   Using plants that have the ability to transfer nitrogen to other plants via the soil. 

5.  Improved pollination.  Specific species of pollinators are known to pollinate specific crops.  Planting companion plants that attract these insects can improve pollination of the target crop.

6.  Improved biological control.  Companion plants can provide habitat for insects or other animals that prey on harmful insects. “Banker plants” can provide habitat for these beneficial insects after one wave of pests has been dealt with, while they wait for another wave to arrive

7.  Improved aesthetics.  Most people simply find a mixture of plants more visually appealing than monocultures.

Specific Combinations to Try

1.  For improved soil fertility and structure. 

A.  Cover crops—warm season and cool season crops that are planted but not harvested, for example:  oats, buckwheat, winter rye, crimson clover, winter wheat, cowpeas/southern peas 

B.  Nitrogen transfer—garden beans + potatoes, fava beans + sweet corn, cowpeas + peppers and other tall transplants, peas + lettuce, edamame (edible soybeans) + fall greens 

C.  Breaking up heavy or compacted soil—buckwheat, forage radish, turnip (especially the kinds with long, tapered roots).

2.  For weed management

A.  Living mulches—Crimson clover + cole crops, medium red clover + winter squash, white clover + strawberries or blueberries, white clover + tomatoes, peppers, eggplants, and other tall vegetables, subterranean clover + many vegetables, oats + tall vegetable crops or berries, winter rye + asparagus, cowpeas + peppers, yellow mustard + summer squash  

B.  For allelophathy benefits—oats and winter rye work for many crops that are transplanted into these freshly killed crops. Oats works especially well for sweet potatoes. Cucumbers + taller vegetables, rapeseed (canola) + potatoes,

3.  For support and structure

Living trellises—corn + pole beans, sunflowers + mini pumpkins, broomcorn + edible bottle gourds, amaranth + chayote, quinoa + bitter melon, sunchokes + cucamelons, orach + fall peas, tithonia + Malabar spinach, kiss-me-over-the-garden-gate + gherkin cucumbers, sorghum + asparagus beans, okra + currant tomatoes, tree kale + runner beans

4.  Pest management

A.  Trap cropping—cabbage + collards to lure diamondback moths, tomatoes + cowpeas to lure Southern green stink bugs, various squash + blue hubbard to lure squash bugs and squash vine borers, bell peppers + hot cherry Peppers to lure pepper maggots, various vegetable + radish or Pak choi to lure flea beetles, cole crops + Chinese mustard greens to lure flea beetles, various vegetables + mustard greens to lure harlequin bugs, strawberries + alfalfa to lure lygus bugs

B. Masking host plants—Peppers + alliums (onion family) for green peach aphids, zucchini + nasturtiums for squash bugs, chines cabbage + green onions for flea beetles, tomatoes + basil for thrips, collards + calendula for Aphids, potatoes + tansy or catmint for Colorado potato beetles

C. To interfere with egg-laying—Cole crops + sage, dill, chamomile, or hyssop for cabbageworms, tomatoes + basil for hornworms, tomatoes + thyme or basil for yellow-striped armyworm, cole crops + thyme for cabbageworm and cabbage loopers, onion and cole crops + marigolds for onion root maggot fly and cabbage root fly, cabbage + white clover for cabbage root maggot flies

D.  To impede pest movement

a.  Hedgerows--wide, linear, permanent plantings composed of a mixture of trees, shrubs, grasses, and perennials that are closely spaced and selected to provide a succession of blooms and a variety of textures and growth habits.  They may also serve a variety of other good purposes.

b.  Low-growing cover crops for soil-dwelling pests—crops that block access to the soil for those pests that lay eggs, pupate, or dwell in the ground

5.  Disease management

A.  Cover crops and living mulches—potatoes + oats or winter rye for verticillium wilt, potatoes + brassicas for potato scab, cauliflower and lettuce + brassicas for verticillium wilt and sclerotina stem rot, tomatoes + hairy vetch for managing foliar diseases, watermelons + hairy vetch for reducing fusarium wilt, beans + winter wheat or winter rye to manage root rot

B.  Improve air circulation—plant in “layers” to facilitate good air circulation:  Tall plants such as tomatoes, peppers, and okra are surrounded by or interplanted with lower growing varieties such as carrots, beets, and bush beans.

6.  Biological controls (focuses on a natural balance between pests and predatory insects)

A.  To attract beneficial insects:  Lettuce and other greens + dill and fennel for aphid control, eggplants + dill or cilantro to control Colorado potato beetle, cole crops + black-eyed susans and cosmos for aphid control, various vegetables + carrot and mint family herbs to control caterpillar pests, lettuce or grapes + sweet alyssum for aphid control, cole crops + lacy phacella for controlling cabbage aphids and other pests, many crops + crimson clover to control thrips

B.  For habitat creation:  broccoli + crimson clover to attract predaceous spiders, lettuce + bunchgrasses to attract ground beetles, low-growing plants to attract ground-dwelling beneficials, hollow-stemmed perennials for winter habitat, hedgerows of closely planted, permanent woody vegetation

7.  Pollination (especially by native bees, since the European honeybee is far more difficult to manage)

A.  To improve pollination:  tomatoes, peppers, and eggplants + large or hooded (snapdragons) flowers to attract bumblebees, blueberries + crimson clover to attract bumblebees, squash + more squash to attract squash bees, flowering herbs + annuals to attract sweat bees, greens and root crops + cosmos, sunflowers, and daisies to attract small native bees, flowering plants to attract mason bees and mining bees to fruit trees,

B.  To create nesting habitat for pollinators (especially tunnel and ground nesting bees):  be smart about garden cleanup, leave 12 inches of stubble when removing plants, eliminate pesticide use, leave dead trees and branches if not a hazard, leave some bare ground, go no-till

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