Companion Planting Beginner’s Guide

Matthew W. Williams, M.S. Crop Science

Companion planting, what is it?  Well, it can be a bit controversial, especially in the scientific community.  Some say it is false, some avoid using the term altogether, some find there to be definite benefits, and some follow these strategies as gospel.  Regardless of how it’s perceived, at its core, companion planting is a type of polyculture.  Multiple species are grown in close proximity to one another as opposed to monoculture which is what we see in much of our conventional agriculture with vast acreages of a single crop species and variety.

For the scope of this guide, we’ll discuss companion planting as the intentional planting of one (or more) species for the benefit of another.  We won’t interpret the term companion planting as mutually beneficial for both species.  Interplanting or intercropping is another term that we should familiarize ourselves with.  Interplanting is using multiple species planted in close proximity with the goal of no negative effect on yield or quality of either, but no direct or mutual benefit either. 

There are also plants that should not be planted together, either because they are very competitive for the same resources, susceptible to similar pests and diseases, or due to allelopathic effects.  Allelopathy is when one plant species secretes natural chemicals that either inhibit germination of seeds or growth of nearby plants.  Below you will find an outline of strategies and examples of species that fit each category.

Companion Planting Strategies

• Planting species that repel pests.  This reduces the need for other forms of pest control.
  • Example: Marigolds potential to repel aphids and whiteflies.
• Planting root vegetables to reduce soil compaction and increase aeration.
  • Example: Plant carrots to break up soil and reduce compaction.
• Planting support plants near climbing plants reduces the need for staking and trellising.
  • Example: Plant corn to support beans.
• Planting legumes to fix nitrogen that will be released in the soil, nourishing other plants in following seasons.
  • Example: Planting peas, beans, clovers, or legume inclusive cover crop mixes.
• Planting plants with a large canopy can reduce weeds, shade other nearby plants, and reduce water use.
  • Example: Planting squash or pumpkins for additional soil cover.
• Planting trap crops that attract common pests and then their natural predators (sweet alyssum)
  • Example: Planting Nasturtium that attracts aphids and flea beetles.
• Planting species that attract beneficial insects and pollinators.
  • Example: Planting a wild flower border to attract lady bugs, praying mantis, and pollinators such as bees.

Other reported benefits of companion planting, although the evidence is varied, is higher yield and better flavor in vegetables planted next to compatible plants.  Companion planting also maximizes the use of your garden space.  Below are potential companion plants categorized by their benefit. Use this table as a starting point for further research.

Companion Planting Design Ideas

I’ve experimented a bit with design, have had some great conversations, and read and watched interesting perspectives and certainly can’t cover all of that information concisely, so I’ll focus on a few key principles that seem shared amongst those embracing companion planting.

When designing, consider your overall planting area, think of the type of plant, the size of the mature plant, the seasonality of growth, and the layout.  In regards to the type of plant, it’s nice to be able to have a diverse mix of herbs, flowers, and vegetables that will complement one another.  The YouTube channel Gardenary and Nicole Johnsey Burke do a great job showcasing these key principles in a kitchen garden.

Assuming we’re planting in a standard rectangular raised bed garden, you can begin by choosing flowers and herbs that will remain relatively small.  These will be planted along the outside of the garden bed.  They will largely act to repel pests and attract beneficials while also bringing a great aesthetic.  These may also act as a trap crop.  Any plants that are selected should be adapted to your climate and the time of year that you are planting, keeping the length of the growing season remaining in mind.  The next step is to select and plant a few large plants; these should be planted near the middle of the bed.  Lastly, select medium size plants and fill out the spaces that are remaining.  Other small plants can be incorporated amongst the flowers and herbs.  Green onions are always a great choice to fill in any small gaps that you can see.

Example Planting Plan by type and size.  Mix and Match to fit your preferences.

Interplanting Strategies

• Add fast growing crops next to, or under, longer season crops.  This increases biodiversity and allows a harvest of the fast growing crop while the other species are maturing.
  • Example:  Add lettuce, green onions, or radishes next to or under tomatoes, peppers, cucumbers, or eggplants.
• Use interplanting to fill available space.
  • Example: Plant relatively small species such as green onions, lettuce, or radish where you see gaps that weeds may invade.
• Interplant crops with different rooting structures.
  • Example:  Plant lettuce or basil with their shallow roots with tomatoes that are deeper rooted.
• Split bed interplanting.  Plant two species with similar length to maturity to enhance biodiversity.
  • Example:  Carrots and leeks or spinach and green onion.

Allelopathic Plants

When planting using the principles of companion planting or interplanting, it is important to remember that some plants have allelopathic effects on other plants, meaning that they will reduce their vigor or even seedling emergence.  Below is a list of plants that have allelopathic effects on others:

• Fennel (Suppress growth of nearby plants)
• Sunflowers (Suppress growth of nearby plants)
• Garlic (Only a big deal for sensitive species such as beans, peas, asparagus, and parsley, but can reduce seed germination and root growth)
• Mustard greens (Suppress germination of seeds)
• Radish (Suppress growth of certain nearby plants such as wheat)
• Mint (Suppress growth of nearby plants; spreads rapidly; only plant in containers)
• Walnut trees (Suppress growth of plants, most significant when growing under dripline)

Plants that are often sensitive to allelopathic plants

Note:  Younger plants are typically more susceptible.

• Peas
• Beans
• Tomatoes
• Peppers
• Potatoes
• Corn
• Carrots
• Lettuce

In general, it is best to avoid growing allelopathic plants in close proximity to those that are sensitive.

Others plant species to not grow together

• Onions and legumes
  • Why? Onions release chemicals that inhibit the growth of many legumes such as peas and beans.
• Carrots and dill
  • Why? Dill can attract carrot flies that prey on carrots.  Cross pollination that negatively affects taste can also occur.
• Tomatoes and corn
  • Why? They attract similar pests such as corn earworm and tomato hornworm.  Both also have high nutrient requirements so will compete with one another for water, nutrients, and sunlight.
• Tomatoes and potatoes
  • Why? They are both in the same family and susceptible to the same diseases such as late blight.  They will also significantly compete with one another for water and nutrients.
• Strawberries and brassicas
  • Why? Brassicas such as kale, broccoli, cauliflower, and brussels sprouts will outcompete strawberries for available nutrients in the soil.
• Potatoes and cucumbers
  • Why? Both are susceptible to late blight and potato harvest may disrupt the rootzone of cucumbers.  Additionally, they will compete significantly for available water and nutrients.
• Melons and potatoes
  • Why? Both are susceptible to similar pests such as aphids and cucumber beetles.  Differing growth habits can make potato harvest difficult.

Final Thoughts on Companion and Interplanting

You’re gardening, so you’re already doing something right!  Whether you are a true skeptic, or subscribe to these growing practices wholly, there are a few other considerations.  When growing these high density, high diversity plantings, no matter how they are labeled, it is important to consider how the soil is impacted both during the growing season and after.  During the growing season, monitoring soil moisture levels carefully is essential.  In high density plantings, water use by plants often exceeds what is expected, especially during seasonally hot weather.  Be sure to adjust irrigation practices as necessary to maintain maximum plant health.  Soil nutrients are also consumed at elevated levels in high density, high diversity plantings as the root systems capitalize on multiple regions and depths of the soil.  A pre-plant or post-harvest soil test by MySoil will help guide you to the proper fertilizer and amount for the next season to ensure long-term garden success and nutrient dense food.  There will also be many garden observations made throughout the season.  A journal noting success, failures, and preferences will also prove irreplaceable as your companion and interplanting techniques evolve over the years.

It is clear that there are many realized and potential benefits to having increased diversity and density in your garden.  These benefits range from attracting beneficial insects to repelling pests, suppressing weeds, or providing support for other plants amongst others mentioned earlier.  I encourage you to continue to learn, experiment, and take advantage of intentional diversity in your companion and interplanted gardens.