This is the low-down on kale asparagus from the Heritage Seed Library:

Brassica oleracea convar. acephala and Brassica napus
The hardiest of brassicas, plants are best raised in the early summer (around May) by sowing into a seedbed outside, or in pots. They are then planted out or transplanted in July or August into their final positions at a spacing of 45cm for dwarf varieties, or at 75cm for taller varieties. They conveniently follow an early crop of peas or broad beans. Plant firmly and water in well. Kales need little attention apart from controlling aphids and cabbage white butterfly and will grow well in most garden soils. Harvest in the depths of winter, when little else can be found, by removing a few leaves from each plant. The younger leaves are less bitter. Varieties of kale will cross-pollinate, as well as with cabbages and cauliflowers.

‘Asparagus’ kale
An old variety, listed in Vilmorin’s The Vegetable Garden in 1885. Reputed to be one of the tastiest of kales, and is treated more like a broccoli. The young leaves can be eaten over the winter, but it is better left until the spring when the young flower shoots can be broken off, blanched and eaten like asparagus. Relatively dwarf, hardy and prolific. “