Friday, October 5, 2012

Myths and Facts of Companion Planting

Companion planting is the establishment of two or more plant species in close proximity so that some benefit is derived - usually higher yields and the control of pests. The fundamentals of companion planting have evolved over the years from a combination of historical observations, some unconventional testing and some scientific research. 

Science has consistently shown that many companion plantings offer no pest control benefits when grown under controlled conditions.   Other companion plantings, however, do have chemical components that repel pests, although the effect is often localized and the benefits do not extend to neighboring plants of different species.While research has not verified all the combinations that have been passed down over the years, there is validity with some of the plant mechanisms that create these beneficial associations.

1. Trap cropping – a given pest may have a preference when it comes to hosts. So when collard greens are planted next to cabbage, the collards will draw the diamond back moth away from the cabbage. Thus the collard crop will be heavily infested while the cabbage is hardly touched.

2. Nitrogen fixers – legumes such as peas, beans and clover, with the help of a  bacteria that live in their roots, will capture nitrogen in the soil and convert it to a form that plants can use.   As an example, beans are sometimes interplanted with corn to reduce the need for nitrogen fertilizer applications.

3. Physical spatial interactions – tall sun loving plants may be planted with low growing shade tolerant plants.  Thus, garden space is better utilized, producing higher total yields per area of land. 

4. Biochemical pest suppression – some plants exude chemicals from roots that will suppress or repel pests and even protect some neighboring plants.  The African marigold releases a chemical called thiopene that repels nematodes.  Another example is when rye is mowed and placed in beds as mulch; a chemical is leached from the rye residue and will prevent weeds from germinating but will not harm established vegetables in the bed.

5. Beneficial habitats - Certain plants can be used as insectary plants to attract and support beneficial insects. It is often the immature stage of an insect that provides the vital service of biological control. But the adult stage must eat, too.  Planting a field of a variety of blooming plants will provide nectar and pollen for these adults. They will be drawn to the area, feed, reproduce and remain.

Here are a few specific examples that researchers have identified:
  • Boston fern leaves contain a feeding deterrent effective against the Southern armyworm. 
  • The leaves of a tomato plant contain a chemical that is toxic to some weevil species.
  • The cabbageworm is controlled by planting mint nearby.  Rosemary, sage, peppermint, thyme and catnip are other helpful plants to control cabbageworms and aphids. 
  • Sweet basil (Ocimum basilicum) tends to repel aphids, mosquitoes and mites.  Planting basil among tomato plants controls tomato hornworms.  
  • Thyme planted with cabbage controls cabbage maggots, white cabbage butterflies and imported cabbageworms. 
  • Nasturtiums are often used as a trap crop for aphids. 
In many plant species, science has discovered beneficial attributes in companion planting. Problems sometime arise, however, when garden enthusiasts attempt to apply this information to their garden.  The facts are true, but the application is often misguided.

Here is an example: It is true that certain species of marigold produce chemicals that repel nematodes.  But there are a few important factors to consider. 1) The correct species of marigold must be planted and at the proper high densities. 2) Correctly planted, marigolds will suppress the nematodes and allow for a susceptible crop to be planted, but only for one season.  It is necessary to continue alternating the marigolds with the desired crop.