Garden Companions

Posted by: Micaela Truslove, Broomfield County Extension

The practice of companion planting – interplanting crops that are mutually beneficial to increase the quality and yield of nearby plants – has been practiced for centuries.  Who doesn't remember their grandparents planting marigolds in the vegetable garden to ward off insect pests?  Many people have planted a Three Sisters garden at some point, which utilizes corn as a trellis for beans, beans as a nitrogen fixer for squash and nitrogen-sucking corn, and squash’s large leaves to shade out weeds. Though companion planting has long been a popular practice in the garden, there is little scientific evidence to support many of these plant associations.  However, there are many beneficial ways that plants can be used to help each other out in the garden.

Zinnias attract pollinators, so are a great
companion plant for the garden. (Photo: 
Micaela Truslove)

One such method is intercropping.  Intercropping takes advantage of plants’ different growth rates, sizes and root depths, allowing you to plant more intensively and make the most out of the growing space.  For instance, a common practice is to plant an early crop of lettuce next to tomatoes.  Shallow rooted lettuce will shade out the weeds while deeper rooted tomatoes get started, and will be ready to harvest by the time the tomato vines get too big.  Intensive planting in general helps to shade out weeds, but caution must be taken not to place plants too close together as this can encourage disease.

Somewhat related to intercropping is succession planting.  Planting short season crops ahead of longer season crops in the same place, though not necessarily at the same time, will allow for an early harvest of spring planted vegetables before longer season vegetables are planted out.  Good examples of this are spinach, radishes and peas, which can all be planted in the “shoulders” of the season (early spring and late summer).  These can be succession planted before and after main season crops, such as peppers and tomatoes.  Variety selection is important in succession planting.  Choose varieties that mature quickly to make sure there is enough time to plant all of your successions.  Another way to use succession planting is to plant crops in two or three week intervals to have a continuous harvest and to avoid a glut of any one vegetable (one can only eat or give away so many zucchini).

Trap cropping is also a useful tool in times of heavy insect infestations.  For instance, in a year when flea beetles are bad, planting a sacrificial row of radishes a little way away from your other brassicas, such as broccoli, will lure flea beetles away.  Eric Hammond, the Horticulture Agent for Adams County, noticed that flea beetles really loved his Basket of Gold (Aurinia saxatilis), so this might make a good trap crop for brassicas.  When you notice that certain plants make good companions, write it down so you’ll remember for next year.

Yarrow (Achillea millefolium) is a great example of a flower with

shallow nectaries and small flowers which can be used in the garden

to attract beneficial insects (courtesy of Carol O'Meara, Boulder County
Extension).

Perhaps the most important way you can help your garden is to increase crop biodiversity and use plants that provide a habitat for beneficial organisms in the garden.  Many people don’t want to give up valuable vegetable real estate to plant flowers, but the benefits of doing so far outweigh the space you have to sacrifice.  Mixing plants of different colors, scents and ripening times and avoiding large areas of plants from the same plant family may confuse pests that are looking for a tasty meal. Herbs often have a strong aroma which may confuse insect pests, and they are also beautiful and useful for cooking.

In addition, plants with small flowers and shallow nectaries (think of your dill or fennel after they flower) often attract predators that eat other insects, including lacewings, lady beetles, assassin bugs, tiny beneficial wasps and syrphid (hover) flies.  Letting some of your vegetable plants flower will give you food and the benefit of flowers that attract helpful insects.  Learning what these insects look like through their various life stages will ensure that you are squashing the correct critters and leaving those that serve a beneficial purpose.  Pollinators are also very important as they are the reason many of our vegetables produce fruit.  Planting flowers that attract both honeybees and other bees is a great way to ensure good pollination.

Lady beetles, especially in the larval stage, are
predators of many insect pests. (Photo: Micaela
Truslove)

There are also negative plant associations.  The one to avoid in the vegetable garden is planting crops from the same family in the same location year after year.  Practicing crop rotation will ensure that your soil will remain healthy and disease free for the following planting season.  Plant crops from the same family in a different location every year. This is often very important for tomatoes as there are many soil-borne diseases that affect tomato plants.  If you are planting certain crops together to gain the benefit of the associations they form, just plant that entire block in another location the following year.

Though there is a lot of information out there about companion planting that has not been scientifically proven, there are many ways that you can use beneficial plant associations in the vegetable garden to create a healthier, more productive environment.  And there is no harm in testing out the more anecdotal associations.  Close observation and keeping good written records will help you identify what works best for you.