I dotted two marigolds at the end of each tomato row just to keep the soil clean: when I lifted the first roots in autumn, it was already too late
Two marigolds planted at the end of a tomato row look decorative but do nothing to stop root-knot nematodes from destroying roots underground. This widespread gardening advice fails because most gardeners plant marigolds as ornaments rather than following the science—requiring dense coverage and precise timing to work.