Plants seem to be passive organisms. Plants, however, are active in many ways. They can change the chemical makeup of the soil and can affect the microorganisms that grow there.

The walnut trees in my yard dictate what can be grown around them. The chemicals found in oak leaves slow the development of insects that feed on them.

Sometimes, the effects of plants are positive. Alfalfa, clover and many other legumes improve the soil with the nitrogen they take from the air. Some trees, such as maples, send water to the soil surface so shallower-rooted plants can grow under drought conditions. Acid-loving plants like azaleas grow best under or near pine trees, and there are many plants that thrive in the shade a large tree provides.

Plants that work well together are often referred to as “companions.”

Many plants create chemicals that can discourage insects and diseases. An example of this is marigolds. Some varieties produce a substance that is toxic to some types of nematodes, damaging roundworms that live in the soil. Planting marigolds between garden rows can be beneficial if nematodes are present.

Some plants, particularly herbs, are strongly scented and can confuse insects searching for a host to feed on. Plants like garlic, onions, chives, catnip, basil and mints all generate scents that appear to repel insects or hide the scents that attract insects.

Dill, parsley, angelica, carrots, coriander and parsnips have clusters of small flowers that have strong fragrances. They seem to attract beneficial insects, in particular predatory wasps and flies.

Try some combinations that, according to folklore, are effective companions: Chives can be grown at the base of roses in an attempt to repel aphids; garlic might be planted under peach trees to deter borers; basil planted among tomatoes might repel tomato hornworms; and nasturtiums grown near squash may ward off squash bugs. Radishes make excellent traps for cucumber beetles amid squash and cucumbers, and garden borders planted with thyme or lavender may deter slugs.

I encourage everyone who gardens to try at least one companion planting. Any time we can improve conditions for our plants without using potentially dangerous chemicals, we have done a wonderful thing for our environment.

• The Green Thumb is a column by Tracy’s master gardeners. University of California-certified master gardeners are available to answer gardening questions from 9 a.m. to noon Tuesdays, Wednesdays and Thursdays at 953-6112 or mgsanjoaquin@ucdavis.edu. Questions for Heather Hamilton can be sent to ucmastergardener@gmail.com.