Radishes and Peas
Beneficial Pair — Good Match
We rate the pest-trap claim here as thin traditional lore - aphids aren't documented to strongly prefer radish foliage the way they do nasturtium's, for instance. The germination-marker use is a genuinely useful gardening technique, but it's a practical row-management trick rather than a companion-planting mechanism in the pest or nutrient sense this site otherwise tracks.
Mechanism: Trap Cropping
Some companion-planting sources describe radishes as drawing aphids away from young pea shoots, but the more consistently practical reason gardeners actually sow the two together is different: radishes germinate in days while peas can take one to two weeks in cool spring soil, so a radish sown in the same row marks where the slower pea seeds are before you can see them, letting you cultivate around the row without disturbing pea seedlings that haven't emerged yet.
Radishes and peas are a classic spring pairing, but the reason gardeners actually plant them together has more to do with row management than pest control, whatever the trap-cropping label might suggest.
A Marker Crop, Practically Speaking
Cool spring soil can make pea germination slow and uneven, and it's easy to accidentally hoe or step on a pea row before you can see where the seedlings actually are. A radish sown in the same line pokes up within days, giving you a visible marker for the row's exact position - useful for weeding and cultivating around it - well before the peas themselves have broken the surface.
Spacing
Sow radishes at their normal 2-3 inch spacing directly within the pea row rather than as a separate planting - the point is having them share the row, not adjacent rows, so the fast-germinating radish marks the slower pea line accurately.
Watch For
Pull radishes as soon as they mature, since leaving them in place lets their leaves spread wide enough to compete with young pea shoots for light right when the peas need it most to establish a strong root system before summer heat arrives.
Frequently Asked Questions
Do I need to plant radishes with peas for a pest benefit, or is it just for the row marker?
The germination-marker use is the more dependable reason to do this - it solves a real, common problem with slow, uneven pea germination. The pest-trap claim is much less certain, so treat any pest benefit as a bonus rather than the main reason to pair them.