Kohlrabi (Brassica oleracea var. gongylodes) forms a round, crisp swelling of the stem just above the ground. It is one of the quickest brassicas, ready about eight to ten weeks after planting out, so it suits both spring and autumn crops and fits well between slower vegetables. Sow indoors from February and plant out from April, 25–30 cm apart; hardened-off plants tolerate light frost. For an autumn harvest, sow outdoors in July. Plant shallowly and do not bury the base of the stem. The soil should be moderately heavy, humus-rich and well supplied with nutrients, with pH 6.0–7.0. Keep watering even: dry spells followed by heavy rain make the bulbs split and turn woody. Protect young plants from flea beetles and cabbage root fly with fine insect mesh. Harvest when the bulbs are 6–8 cm across, roughly tennis-ball size, as larger ones quickly become fibrous. Purple varieties tend to be hardier and slower to go woody than pale green ones.