How to Prevent Aphids

You can use different methods to control aphids once they appear in the garden, but it's even better if you can keep them away from your plants in the first place. As the old saying goes, "An ounce of prevention is worth a pound of cure." Keeping aphids out is healthier for your plants and means less work for you.

Aphids are among the most common, and most varied, of garden pests. They all have soft, pear-shaped bodies, but different species target different types of plants. Most attack leaves, stems and flowers. Others, called root aphids, attack the base and roots of the plants.

Aphids are most likely to attack succulent new growth. Often, this new growth is triggered by nitrogen fertilizer. Nitrogen is necessary for plant growth, but too much triggers excessive growth that aphids love. Choose a product described as "slow-release" to limit the amount of nitrogen available to the plants at one time, and follow label directions to avoid overfertilization.

If you amend soil with compost, you can further reduce the amounts of chemical fertilizer. A typical application rate for well-rotted compost in vegetable and flower gardens is 1 to 3 inches spread over the soil and worked in to a depth of 6 to 8 inches. Apply once a year, in the spring, as new growth starts. Use the lower range for composts containing manure, and the higher range for composts made of plant matter.

If you're applying compost every year, you can use the lowest application rate for chemical fertilizers, and increase the time between applications. For example, a continuous release all-purpose plant food with an N-P-K ratio of 12-4-8 has an application rate of 1 tablespoon per 1 square foot every 3 months. If you added compost in the spring, wait until one month after that to make the first application, and apply no more than 1 tablespoon per square foot. Reapply in three to four months.

Aphids can end up in your garden when they are carried in on new plants. Inspect plants carefully before bringing them into your garden. Pay careful attention to the undersides of leaves, keeping in mind that aphids are often green and blend in easily.

When plants in your garden are actively growing, inspect them weekly to make sure aphids haven't appeared. If you catch aphids early, they are much easier to get rid of. A strong spray of water from a hose is enough to remove a small aphid infestation. Aphids can also show up on weeds, so keeping up with the weeding in a garden helps prevent infestations from getting out of control.

If you can attract aphids' natural predators to the garden, you're much less likely to have a bad aphid outbreak. Beneficial insects that kill aphids include parasitic wasps, syrphid fly larvae and lacewing larvae. Lady beetles also eat aphids, but they are attracted to the aphids, not to other plants in the garden.

Planting host plants that attract these insects is the best way to bring them into the garden. Adult parasitic wasps, syrphid flies and lacewings are attracted to nectar-producing plants. Stagger bloom times to make sure the adults that lay aphid-eating larvae always have a steady food supply.

Basket of Gold (Aurinia saxatilis 'Gold Dust'). U.S. Department of Agriculture plant hardiness zones 4 through 7. Blooms mid- to late spring.

Creeping thyme (Thymus serpyllum). USDA zones 4 through 8. Blooms late spring and early summer.

English lavender (Lavandula angustifolia). USDA zones 5 through 8. Blooms early to midsummer.

Dill (Anethum graveolens). USDA zones 2 through 11. The annual looms from midsummer through fall. 

When you're trying to encourage beneficial insects, avoid pesticide use as much as possible, and read labels carefully to choose the least toxic option.

Aphids that attack plant crowns and roots are harder to notice and keep out of the garden. If they appear one growing season, there are steps you can take to prevent them showing up next year. Till soil before planting to kill aphids in the soil, and rotate crops. Root aphids are crop-specific, so if you have problems with root aphids on something like annual lettuce (Lactuca sativa) one year, you can safely plant a different vegetable in that location next year.