Companion Planting Guide

Before selecting companions, know what you are trying to accomplish:

**Pest disruption:** Some plants repel or confuse pest insects through scent or volatile compounds. Others attract beneficial predatory insects. African marigolds (Tagetes erecta) have well-documented nematode-suppression properties when grown as a dense planting.

**Pollinator attraction:** Flowering companions like dill, fennel (keep it isolated — see Step 6), and borage attract bees and other pollinators, which improves fruit set on cucurbits, tomatoes, peppers, and beans.

**Space efficiency:** Tall plants provide shade for heat-sensitive shorter plants. Sprawling plants cover bare soil, reducing weed pressure. Trellised crops free up ground-level space for low-growing crops.

**Nutrient sharing:** Legumes (beans, peas, clover) fix atmospheric nitrogen through root-dwelling rhizobacteria. This nitrogen becomes available to neighboring plants — most significantly after the legume plant is cut down or incorporated into soil.

The Three Sisters — corn, beans, and squash — is the most well-documented companion planting system in North American agriculture. Here is how it works:

**Corn** provides a vertical structure for beans to climb, eliminating the need for a separate trellis.

**Beans** (pole varieties) fix nitrogen from the air into soil-available form through symbiotic rhizobacteria on their roots. The corn benefits from this nitrogen, and residual nitrogen improves soil for the following season.

**Squash** spreads across the ground between the corn stalks, shading the soil to retain moisture, suppress weeds, and deter pests with its rough, scratchy leaves.

**How to plant:** In Anderson, CA, direct sow corn in mid-April after soil reaches 60°F. When corn is 6–8 inches tall (about 2 weeks after emergence), plant 3–4 bean seeds around each corn stalk at 6-inch spacing. One week later, plant squash in the spaces between corn hills. Do not plant all three at the same time — the corn needs to establish before beans are added, or the beans will outcompete it early.

Plant corn in blocks of at least 4 rows for wind pollination. A single row will produce poorly pollinated ears.

African marigolds (Tagetes erecta) — not French marigolds (Tagetes patula) — have documented root exudates that suppress soil-dwelling nematodes (Meloidogyne spp.) when planted densely as a cover crop or border planting. French marigolds produce some of the same compounds but at lower concentrations.

For nematode suppression, research from UC Cooperative Extension indicates that the marigolds need to be planted at high density (not just a few plants as border decoration) and allowed to grow for at least 60 days before the bed is replanted. Using them as an off-season cover crop before a summer tomato bed is one of the most practical applications.

For in-season planting, place African marigolds at the edges of tomato and pepper beds. They also attract aphid predators (ladybugs, lacewings, parasitic wasps) through nectar and provide a visual indicator of aphid pressure — aphids will often colonize marigolds, making them easy to spot and remove.

**Do not confuse this with a magic cure.** Marigolds planted as a few decorative borders will not meaningfully reduce nematode pressure. Dense, whole-bed plantings for a full season are required for measurable suppression.

Aromatic herbs planted among vegetables can interfere with pest insect host-finding. The evidence varies by combination:

**Basil near tomatoes** is widely recommended and widely practiced. Controlled studies show mixed results — the mechanism (scent disruption for thrips and aphids) is plausible but not consistently proven. It does not hurt, and the basil is useful in the kitchen. Plant after tomato transplants are established.

**Dill and fennel:** Both attract beneficial parasitic wasps and predatory insects. **Important:** Fennel (Foeniculum vulgare) is allelopathic — it produces root and leaf exudates that inhibit germination and growth of most vegetable crops. Plant fennel in its own isolated container or area, well away from vegetable beds. Dill is not allelopathic and is safe to interplant.

**Catnip (Nepeta cataria):** Several studies have found catnip volatile compounds effective at repelling aphids, Japanese beetles, and some other pests. It is a vigorous grower — contain it or it will spread aggressively.

**Nasturtiums:** Often used as trap crops — aphids, whiteflies, and caterpillars are attracted to them preferentially. Plant at the edges of beds and check weekly. When pest populations are high on nasturtiums, remove and dispose of the plants (do not compost them) before pests move to your food crops.

In Anderson, CA, summer temperatures regularly exceed 105°F. Strategic use of tall crops as shade sources can extend the productivity of heat-sensitive companions:

**Corn or sunflowers on the west or south side of lettuce:** In late spring, before heat shuts down cool-season crops, tall corn or sunflowers planted on the side of the bed facing the afternoon sun can provide shade during the hottest part of the day. This extends lettuce harvest by 2–3 weeks in a hot spring.

**Trellised tomatoes or beans over spinach or cilantro:** A trellis on the south side of a low-profile bed provides dappled shade. This works best in spring before heat intensifies.

Note: In peak summer (July–August), most cool-season crops will not survive even with shade. The goal is to extend the shoulder seasons, not to grow lettuce in August.

Fennel (Foeniculum vulgare) is one of the most commonly misused companion plants. Many gardeners plant it expecting it to attract beneficial insects (it does attract swallowtail butterfly larvae). The problem: fennel root exudates and leaf litter inhibit the germination and growth of most vegetable crops, including tomatoes, peppers, lettuce, and brassicas.

Do not plant fennel in vegetable beds. Instead: - Grow fennel in a large container near the garden - Plant it at least 3–4 feet from vegetable beds in a dedicated herb area - If you grow it in the ground, do not till fennel roots back into vegetable bed soil

The beneficial insect attraction is real — just manage the allelopathy risk.

Draw a simple bed layout before you plant. Note: - Which crops are tall (corn, tomatoes, sunflowers) vs. low-growing (basil, lettuce, spinach) - Which direction afternoon sun comes from (west to southwest in California) - Which crops need pollination (cucurbits, corn) vs. which do not (leafy greens, root vegetables) - Where you will establish a permanent marigold border - Where your fennel container will sit (isolated)

A bed layout done in advance avoids the common mistake of planting tall crops where they shade shorter crops that need sun, or planting allelopathic plants where they harm neighbors.