Spacing: Grow like broccoli, 3 to 4 feet apart.
Sowing date: It is easiest to sow in April or early May, minimally fertigate a somewhat gnarly plant through the summer, push it for size in fall and winter, and then harvest it next March. With too early a start in spring, some premature flowering may occur in autumn; still, massive blooming will resume again in spring.
Overwintering green Italian types such as ML423 (TSC) will flower in fall if sown before late June. These sorts are better started in a nursery bed around August 1 and like overwintered cauliflower, transplanted about 2 feet apart when fall rains return, then, pushed for growth with extra fertilizer in fall and winter.
With nearly a whole year to grow before blooming, Purple Sprouting eventually reaches 4 to 5 feet in height and 3 to 4 feet in diameter, and yields hugely.
Irrigation: It is not essential to heavily fertigate Purple Sprouting, though you may G-R-O-W enormous plants for their beauty. Quality or quantity of spring harvest won't drop one bit if the plants become a little stunted and gnarly in summer, as long as you fertilize late in September to spur rapid growth during fall and winter.
Root System Vigor in the Cabbage Family
Wild cabbage is a weed and grows like one, able to successfully compete for water against grasses and other herbs. Remove all competition with a hoe, and allow this weed to totally control all the moisture and nutrients in all the earth its roots can occupy, and it grows hugely and lushly. Just for fun, I once G-R-E-W one, with tillage, hoeing, and spring fertilization but no irrigation; it ended up 5 feet tall and 6 feet in diameter.
As this highly moldable family is inbred and shaped into more and more exaggerated forms, it weakens and loses the ability to forage. Kale retains the most wild aggressiveness, Chinese cabbage perhaps the least. Here, in approximately correct order, is shown the declining root vigor and general adaptation to moisture stress of cabbage family vegetables. The table shows the most vigorous at the top, declining as it goes down.
Adapted to dry gardening Not vigorous enough
Kale Italian broccoli (some varieties)
Brussels sprouts (late types) Cabbage (regular market types)
Late savoy cabbage Brussels sprouts (early types)
Giant "field-type" kohlrabi Small "market-garden" kohlrabi
Mid-season savoy cabbage Cauliflower (regular, annual)
Rutabaga Turnips and radishes
Italian Broccoli (some varieties) Chinese cabbage
Brussels Sprouts
Sowing date: If the plants are a foot tall before the soil starts drying down, their roots will be over a foot deep; the plants will then grow hugely with a bit of fertigation. At Elkton I dry garden Brussels sprouts by sowing late April to early May. Started this soon, even late-maturing varieties may begin forming sprouts by September. Though premature bottom sprouts will "blow up" and become aphid damaged, more, higher-quality sprouts will continue to form farther up the stalk during autumn and winter.