From one plant we get facsimiles of Kale, cabbage, asparagus and broccoli. It also has edible and aesthetic landscaping value. Sounds too good to be true, which leaves one to ponder why it was never commercialized since its been known since at least the dark ages. It's one major drawback is that it has an extremely limited shelf life - very short storage period and does not lend itself well to shipping.
Sea Kale can be grown from divisions planted either in the spring or fall. Root cuttings [thongs] should be dormant and are best started in containers until they produce viable seedlings. They can also be started directly from seeds, unfortunately - like the plant itself, the seeds have a very short shelf life and must be from the current season - even then the germination rate is usually below standard so be sure to plant more than you will need.
Direct seeding into the garden is feasible in Spring, keep the seed bed and subsequent seedlings well watered for best results. Although direct seeding is works best results come from seeds started indoor in peat pots or cell packs and transplanted outdoors at an opportune time, soil temperatures should be at the very least 40 - 45 F.
The seeds have a hard outer shell known as a pericap, some people will remove this prior to planting. If you have all day to fiddle fart around with the seeds - go for it, although you'll certainly destroy some in the process. I prefer to make more efficient use of my time and soften the seed casings by pre-soaking them in salt water over night. The salt should be non-iodized. Sea Kale due to its lineage as a sea side plant will tolerate the salt - don't try this with common vegetable seeds.
In early spring the sea kale plants will produce new shoots. If you plan on harvesting the shoots like asparagus they should be 'blanched' in much the same fashion that white asparagus is.
Unblanched sea kale shoots are bitter and unpalatable. The blanch them is to deny the emerging shoots access to as much light as is possible, thereby blocking its ability to produce chlorophyll - the compound which gives plants their green color and in the case of sea kale shoots their bitter taste. Anything that blocks the sunlight will suffice. An unused planter flipped upside down, a plastic tub, and so forth placed over the plant for up to a month. The plant will continue to grow under the pot or bucket you used - it will simply remain white with some purple highlights.
Leaves are best harvested when small and subtle in the Spring . Older leaves are only palatable when cooked as they become hard and fibrous.
Shoots should be harvested in Spring when soft, tender and tasty.
Flowering heads are harvested in Summer, in much the same fashion as broccoli.
Roots are harvested when the plant goes dormant in late fall or early winter. Only harvest the small outer roots, not the central tap root if you wish the plant to come back the following spring.