Tomatoes and Basil
Beneficial Pair — Good Match
The pest-masking mechanism is plausible and consistent with how scent-masking works in other documented intercropping pairs, but it hasn't been isolated and confirmed in a controlled trial specific to basil and tomato pests. The far more commonly repeated claim - that basil improves a nearby tomato's flavor - is traditional lore with no controlled evidence behind it at all; we mention it because it's the reason most gardeners plant this pairing in the first place, not because we think it's true.
Mechanism: Pest Masking (Scent Confusion)
Basil's strong, volatile scent is the mechanism most often cited for this pairing - the idea is that it masks the chemical cues certain egg-laying moths (including the tomato hornworm's parent moth) use to locate a tomato plant to lay eggs on, so a basil-scented tomato is a slightly harder target to find by smell. This is biologically plausible - scent-masking against host-seeking insects is a real phenomenon in other documented intercropping systems - but it hasn't been isolated and confirmed specifically for basil and tomato hornworm moths in a controlled trial we're aware of, which is why we don't rate it higher than mixed evidence.
Tomatoes and basil are planted together more than almost any other pairing in vegetable gardening, and the two reasons usually given for it carry very different levels of actual support: a scent-masking effect against certain tomato pests, which is plausible but unconfirmed, and a flavor-improvement effect on the tomato itself, which is popular folklore with essentially no controlled evidence behind it.
The Flavor Claim, Specifically
The idea that basil growing near a tomato plant somehow changes the flavor of the tomato's fruit is one of the most widely repeated claims in companion-planting folklore, and it's worth being direct about it: there is no controlled study we're aware of that has isolated and confirmed this effect. Basil and tomato roots don't share tissue, and there's no established biological pathway by which a neighboring plant's aromatic oils would migrate into a tomato fruit's flavor compounds through the soil. This doesn't mean the pairing is useless - it means the reason most gardeners give for it doesn't hold up, even though the pairing itself is still a reasonable one for other reasons.
A Better Reason to Plant Them Together
The more defensible reason is practical rather than mystical: basil is a genuinely useful low, bushy living mulch at the base of a tall staked tomato, shading the soil surface and reducing moisture loss between waterings, without competing meaningfully for the tomato's deeper root zone. It's also simply a good use of space - the area directly around a staked tomato's base is otherwise often left bare, and basil fills it productively while asking for very similar sun and water conditions, which matters more for a successful pairing than most gardeners give it credit for. Two crops with genuinely different water or sun needs make poor companions regardless of any pest-related benefit, and tomatoes and basil happen to want almost exactly the same growing conditions.
What We'd Actually Recommend
Plant basil at the base of your tomatoes if you were going to grow both anyway and want to make efficient use of bed space - there's no real downside once you account for airflow (see Watch For below), and the scent-masking benefit, while unconfirmed, costs nothing to hope for. Don't plant it specifically because you expect it to change how your tomatoes taste; if flavor is your goal, that comes down to variety selection, ripeness at harvest, and growing conditions like consistent watering far more than any companion plant.
Spacing
Basil is shallow-rooted and low-growing (10-12 inches), so it tucks in easily at the base of a staked or caged tomato without competing for the tomato's deeper root zone. Give it roughly 10-12 inches from the tomato's main stem so both get enough light and airflow - basil crowded directly against the stem is more prone to fungal issues from reduced air circulation.
Watch For
Basil is frost-tender, just like tomatoes, so plant them out together after your last frost date rather than starting basil early. Watch for powdery mildew on basil in humid climates - it spreads fast in the same dense, low-airflow conditions that basil crowded against a tomato stem creates.
Frequently Asked Questions
My tomatoes are already in the ground without basil - is it worth adding some now?
Usually yes, if you do it soon. Basil transplants (not seed, this late) establish quickly and can still be tucked in at a tomato's base weeks after the tomato itself went in, without disturbing an already-settled tomato root system. The main thing you'd lose by waiting is total ground-cover time before frost ends both plants' season together.
Will pinching basil back for pesto reduce whatever benefit it's giving the tomato?
No - regular pinching keeps the plant bushier and more vegetative, which if anything means more scent-producing foliage near the tomato's base than if you let it bolt and flower. Harvesting basil for the kitchen and getting whatever this pairing offers aren't in tension with each other.