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tomato and basil growing together

A New Approach to Companion Planting: Adding Diversity to the Garden

Publication Date: August 1, 2018 | Prepared by: Dr. Judy Hough-Goldstein, Delaware Master Gardener


Do tomatoes love basil but hate brussels sprouts? Traditional companion planting, which involves  planting different types of plants together or in close proximity, makes many such statements, often  based on little more than folklore. In recent decades, however, scientists have found that in fact there  are definite benefits to adding diversity to your garden, primarily because certain plants attract and  support beneficial insects that either help control pests or help pollinate your crops.


Why add diversity? 

  • Resources from flowers, i.e. nectar and pollen: 
  • Attract and support predators and parasitoids (natural enemies) for pest control. Some tiny  parasitoids require nectar or another source of sugar, and abundant nectar improves their  survival and increases reproduction. Syrphid or hover flies feed on nectar and pollen, and lay  hundreds of eggs near soft bodied insects such as aphids. The eggs hatch into larvae that are  voracious feeders on those small pests. Lacewings and ladybird beetles are also attracted to  flowers and can effectively control aphids. 
  • Attract and support pollinators, providing nectar and pollen that bees and butterflies need. • Some flowers support alternate prey for natural enemies. For example, marigolds often  support numerous tiny pest insects called thrips, which in turn are fed on by tiny voracious  predatory bugs, Orius species, also known as minute piratebugs. The Orius may also patrol  nearby crop plants, reducing populations of soft-bodied pests.   
  • Marigolds are also well-known for their ability to control nematodes due to the toxic chemicals  exuded by their roots; however, this works better when marigolds are planted as a cover crop,  with their residues tilled into the soil before the crop is planted.
  • The presence of different plant species grown together can disrupt the ability of pests to find  their host plants. Plant-feeding insects, especially those that only feed on one or a few types of  plants, find their host plants by sight and smell, and these cues can be disrupted by  surrounding plants, especially aromatic or bushy plants. 
  • Adding ground-level complexity, for example strips of grass, weeds, or cover crops, can  increase populations of ground-dwelling generalist predators, such as ground beetles and  spiders, which can feed on plant-feeding insects, especially those that spend a part of their  lives in the soil. Grassy “beetle banks” alsoprovide sheltered overwintering habitat for insect predators. 
  • Caution: although there are many studies showing that diverse plantings increase populations  of natural enemies, this does not always translate into fewer pests on the desired crop plant,  since the predators and parasitoids may not disperse to where they are needed. You will still  need to check your plants frequently for unwanted pests, and control them in some other way  if the good bugs are not effectiveenough! 
  • In addition, the added plants will compete with the crop for light, water, and nutrients, and  therefore crop yield may be reduced compared to a monoculture of the crop. Select  companion plants that are not overly competitive with the maincrop. 
  • Other examples where diverse plantings make sense. Plants with different characteristics can  be used to support soil nutrition and better yield in a small space. For example, legumes, such  as alfalfa, clover, peas, and beans, fix nitrogen and can be planted in rotation with non legumes; deep-rooted plants can be grown with shallow-rooted plants; can provide shade for shade-loving plants and also help to reduce weeds. A classic example of  this type of companion planting is the so-called “three sisters” planted together by Native  Americans. Corn, pole beans, and squash were planted together, with corn providing support  for the beans and the large squash leaves suppressing weeds. 


How to add diversity? 

  • Combine crop plants with other annual or perennial plants in the vegetablebed: 
  • Herbs are desirable crops themselves, and often produce small flowers with exposed nectaries that attract and support predators and parasitoids. They also tend to be aromatic. Herbs that  have been shown to attract natural enemies or repel herbivores in at least some studies  include basil, coriander, rosemary, lavender, parsley, borage, dill and chives.
  • Annual flowers can be planted in the beds around vegetables. In the Master Gardener  Vegetable and Fruit Demonstration Garden (VFDG) at the New Castle County Cooperative  Extension Office, sweet alyssum has been planted within vegetable beds for several years to  attract and support beneficial insects. Alyssum is inexpensive, easy to grow, low growing and  compact (so it doesn’t compete with the vegetables), has a long growing season, and doesn’t  leave significant biomass at the end of the season. It can be grown from seed, but at the VFDG plants are typically purchased, for convenience and quick establishment. Numerous syrphid  flies (mentioned above) can be seen swarming about the flowers in the spring. Otherpopular choices are marigolds and nasturtiums. Some popular annual plants such as pansies, however,  have been selected primarily for large colorful flowers and may not produce the nectar and  pollen needed by beneficial insects. 
  • Plant flowering native perennials in a bed near the garden. The VFDG also has a native  perennial bed, consisting of various species that bloom at different times and are known to  attract and support beneficial insects (seeR ESOURCES for plants to consider). 
  • Plant flowering shrubs and trees in a nearby hedgerow. If you have room for trees and shrubs,  this is a great way to add additional flower resources, in addition to berries and habitat for  birds. Trees are often the first source of flowers in the spring (see RESOURCES).
Syrphid or hover fly on alyssum flower
Syrphid or hover fly on alyssum flower
Lacewing on alyssum flower
Lacewing on alyssum flower
Marigolds planted with squash
Marigolds planted with squash
Orius predatory bug feeding on thrips
Orius predatory bug feeding on thrips
Basil planted with tomatoes
Basil planted with tomatoes
Alyssum in the Master Gardener Vegetable and Fruit Demonstration Garden in Newark, DE
Alyssum in the Master Gardener Vegetable and Fruit Demonstration Garden in Newark, DE
Perennial flower border in the Master Gardener Vegetable and Fruit Demonstration Garden in Newark, DE
Perennial flower border in the Master Gardener Vegetable and Fruit Demonstration Garden in Newark, DE


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