Why tomatoes love basil and hate cucumbers

When I say "companion planting", most people think of moon phases, healing crystals, and cosy esoteric shops in Bern.

That's because companion-planting books are often written in that register. "The tomato loves the basil" — as if the tomato had feelings and sent little messages to the basil. No wonder rational gardeners skip that chapter.

But companion planting is neither esoteric nor mystical. It's plain chemistry and biology. And if you understand three rules, you can forget the rest.

Rule 1: Plants compete for the same nutrients

Every plant draws certain nutrients from the soil. Tomatoes are what you call heavy feeders — they consume a lot of nitrogen, potassium, and phosphorus. So do cucumbers. So do zucchini.

If you plant three heavy feeders side by side, they compete for the same food. Each one gets less, all of them grow worse. Not because they hate each other. Because there simply isn't enough for everyone in the soil.

Good combination: heavy feeder + light feeder. Tomato (heavy) + lettuce (light). The lettuce takes almost nothing, and better still — it grows low enough that the tomato shades it.

Rule 2: Some plants share the same diseases

That's the story with tomatoes and cucumbers. Both are susceptible to mildew and certain fungi. If you plant them side by side and one gets infected, it jumps straight across. No mysticism — just spore geography.

So: tomatoes and cucumbers never directly next to each other. At least one metre apart, with a plant from a different family in between (Swiss chard, lettuce, rocket).

Same applies to potatoes and tomatoes. Both nightshades, both prone to blight. If one gets sick, the other dies with it.

Rule 3: Some plants really do help each other

Now the part that sounds like esoterica — but is actually biology.

Tomato + basil: basil releases essential oils that keep certain pests (whiteflies, thrips) away. Tomatoes tend to attract those pests. That's not a "feeling", that's measurable chemistry in the air around the plants.

Carrot + onion: carrots get attacked by the carrot fly, onions by the onion fly. Both flies are confused by the smell of the other plant. Plant them together and you drastically reduce both pests.

Corn + bean + squash (the "three sisters"): beans fix nitrogen from the air and fertilise the soil. Corn provides a natural climbing trellis. Squash shades the ground and keeps weeds down. That isn't an indigenous myth — it's documented permaculture practice with 500 years of results behind it.

The most important "love-hate" pairs

Here's the practical short list for your next raised bed:

✅ Yes together	❌ No together
Tomato + basil	Tomato + cucumber
Tomato + parsley	Tomato + potato
Carrot + onion	Onion + bean
Lettuce + radish	Cabbage + strawberry
Cucumber + dill	Cucumber + tomato (see above)
Zucchini + nasturtium	Zucchini + pumpkin

That's not the whole list. But if you just remember these six pairs, you'll avoid 80% of the most common companion-planting mistakes.

Or — and this is the reason for Erntezeit —

Or you don't remember anything. You tell Erntezeit: "I want tomatoes, cucumbers, carrots and basil" — and Erntezeit automatically works out which ones can live side by side, which ones need distance, and where to position them in the bed.

The Erntezeit companion-planting database has over 300 pairs in it. Nobody should have to memorise that. That's exactly why computers are useful: they forget nothing.

🦔 Stachi