In vegetable gardens, as in nature, a variety of plants will support a variety of wildlife, and diversified plantings will promote a healthy balance between those organisms that benefit crops and those that gardeners would rather do without. Several techniques for "mixing it up" in the home vegetable garden, including companion planting, succession planting, and the use of resistant varieties, can help to confound pests and diseases, and may at the same time increase yields per square foot. By providing natural pest control these techniques uphold biodiversity, and along with variety they can add color, beauty, and flavor to the home garden.

Although the classic image of a vegetable garden is based on neat rows of plants marked by seed packets on stakes, vegetable gardens can be as unique and personalized as ornamental plantings, designed to suit the garden site and the aesthetic and culinary tastes of the gardener. As long as crops can be weeded, watered, and harvested, vegetables can be grouped in all sorts of symbiotic arrangements, with other vegetables or other types of plants, either in one central location or in fertile pockets here and there.

Companion Planting

Companion planting is a general term used to describe the productive combination of two or more different plants in the garden. Different crops might be interplanted, or a certain type of plant might be placed in strategic spots around the garden. There are several ways in which plants in close proximity may benefit other plants, some which have been clearly demonstrated, and some which are based more on anecdotal evidence than on hard data.

Certain plants make good companions simply because their physical needs are compatible. For example, shallow-rooted vegetables such as leaf lettuce can share soil with deep-rooted crops such as carrots, and carefully situated corn can provide a little welcome shade for cucumbers or support for pole beans.

Other plants make good companions because they help to repel pests that bother their garden neighbors. There seem to be a number of ways in which plants discourage pests, and the ways in which they do so are not completely understood. The scent of various plants, particularly aromatic herbs and members of the allium family (which includes garlic, chives, leeks, shallots, and onions), seems to act as a powerful deterrent to some bugs, though it's not clear whether the affected bugs shun or are simply confused by the strong odors of these plants. For example, garlic is reputed to repel a range of insects, including Japanese beetles; spearmint is touted as a defense against aphids; and sage, rosemary, and thyme are used to drive cabbageworm butterflies away. There are also certain plants that exude a selectively distasteful substance through their roots; in this way, some varieties of marigolds control nematodes. Colors in the garden, and the texture of protective foliage, may also affect which creatures feel welcome.

Sometimes, instead of repelling pests a plant can act as an attractive "trap" for insects that would otherwise tend to damage a more valuable crop. For instance, dill can be used to lure hornworms away from tomato plants.

Happily, even while helping to control certain critters, most herbs and flowers will attract bees and other beneficial predators and pollinators. Pollen- and nectar-producing plants can be a timely source of food for beneficial insects. In some cases, pollen is an alternate food source; ladybird beetles may look for pollen when their preferred diet of aphids is depleted. In other cases, an insect requires a different diet at different points in its life cycle. The braconid wasp is one of several species of wasps and flies that parasitize an array of destructive larvae when immature, but feed on nectar as adults. So a good source of pollen and nectar in the vegetable garden can help to keep multiple generations of desirable insects nearby.

Companion planting often leads to another bonus for vegetable gardeners: an increased variety of flavors available to harvest. There are even some edible flowers, such as nasturtiums, that are widely recommended as companions and that make an interesting addition to a freshly picked salad. Be aware, however, that a few repellent plants are toxic when ingested and should only be planted with extreme caution and concern for unsuspecting nibblers, especially children and pets.

Succession Planting

Succession planting involves planting crops at intervals during the growing season. It can refer to staggering the planting of a certain crop (perhaps in different parts of the garden), so that it will be present at various stages of maturity at any given time. It can also refer to planting a series of different crops in the same plot of soil.

Having a crop mature over a stretch of time is great for gardeners who like to extend the pick-and-eat season of their produce. It also helps to ensure that at least a portion of the crop will be relatively unscathed by pests and diseases, which usually do the bulk of their damage at certain points during their life cycles to plants that are in a vulnerable stage of development. The specifics vary depending on the plant and the problem, but with close observation over the years gardeners can get pretty good at scheduling a series of plantings for best results. And if a new problem appears, chances are it won't affect all plantings equally.

Following an early, fast crop (like spinach, beets, or radishes) with a later crop (such as corn, beans, or cucumbers) is another way of tackling pest problems naturally while optimizing overall yields. Many pests have a soil-borne stage, so cultivation between plantings can be used to interrupt their life cycles. Also, replacing a preferred host plant with a less desirable one may cause pests to move on; for that reason it's usually best to follow any given crop with one from a different family. Another reason to rotate crops is to vary the concentrations of microorganisms (which are closely associated with plant roots) in the soil. With careful planning, gardeners can alternate plants that affect the soil in complementary ways, and so help to keep the soil ecosystem robust and balanced.

Diversified Crop Variety Planting

Planting a wide assortment of crop varieties helps to keep vegetable gardens healthy and productive. Different varieties of the same crop often mature at different rates, so even with synchronized plantings the effect can be similar to, though less pronounced than, that with succession planting. Some varieties, particularly heritage and heirloom varieties, offer many deliciously distinctive flavors that aren't usually available in the supermarket. And by choosing to include some resistant varieties of vegetables in the mix, gardeners can help biodiversity by adding another layer of natural, as opposed to chemical, pest control.

Several different varieties of any given vegetable species are usually available to home gardeners, and one or more of them may have a good track record of resistance to particular problems. The resistance may be either natural or the work of plant breeders. Some crop varieties are labeled resistant because insects avoid them: the same characteristics that often distinguish effective companion plants--strong odors, unappealing colors, and unwelcoming textures--often come into play. On the other hand, some plants are unattractive because they are relatively unscented. Insects seeking shelter can be put off by structural features such as a tight husk on an ear of corn, and there are some crops that actually disable insects by either poisoning or trapping them. There are also plant varieties that are considered resistant simply because they can tolerate quite a bit of damage before their yield is significantly affected.

Since gardening conditions can vary so widely, plants may show much better resistance in some environments than in others, so gardeners will usually need to experiment to determine what works best for them. Also, as pests and diseases constantly evolve, a variety that is quite resistant one year may be less so down the road. So even when choosing resistant varieties, diversity is key. Though one variety may be clearly outperforming another at a given point in time, it's impossible to predict all of the variables that can affect the eventual harvest. For instance, a variety that is successfully resisting pests may be hit especially hard by an unusually rainy spell or by a newly introduced disease. Planting several varieties of any one crop lets gardeners hedge their bets in the short term, while promoting and preserving the genetic diversity that will help gardens flourish in the long term.

References

Foster, Catharine Osgood. The Organic Gardener. New York: Vintage Books, 1972.

Lavoipierre, Frédérique. Garden Allies: Braconid Wasps. Pacific Horticulture: The magazine for West Coast gardeners April 2008.

(Note: "Garden Allies" is a regular column in Pacific Horticulture that is packed with information about beneficial garden critters. Here is a list and links to all of the articles in the series)

Teetes, George L., Texas A&M University, 1996. Plant Resistance to Insects: A Fundamental Component of IPM in Radcliffe's IPM World Textbook ed. E.B. Radcliffe, W.D. Hutchison and R.E. Cancelado. http://ipmworld.umn.edu University of Minnesota, St. Paul, MN.

Wright, Mark G. and Michael P. Hoffman. Selection of Vegetables for Intercropping as a Pest Management Strategy. Department of Entomology, Cornell University, Ithaca, NY, January 2001. www.organic.cornell.edu/research/tsfsumms/organicpdfs/7intercrop.pdf

Yespen, Roger B., Jr., et al., eds. Organic Plant Protection: A Comprehensive Reference on Controlling Insects and Diseases in the Garden, Orchard and Yard without Using Chemicals. Emmaus, PA: Rodale Press, Inc., 1976.

Photo Credits

All photos in this article were taken by Martha Goelzer at the Lake Hills Greenbelt Urban Demonstration Garden and Community Gardens, Bellevue, WA.