Seed to Seed Guide: Kale
A Como Community Seed Library guide to growing kale — from transplant to table, and from flower to next year's seed packet.
Kale is one of the easiest brassicas to grow for food, and one of the more forgiving ones to grow for seed — but there's a twist most gardeners don't expect: your kale might be one of two entirely different species. Here's how to grow it for the table, how to grow it specifically for seed, and why that species question matters more than you'd think.
Growing Kale for the Table
Starting Seed
For a spring crop, direct-seed or transplant 4–6 weeks before your last expected frost — kale seed germinates at soil temperatures as low as 40°F. For a fall crop, start seed in June so plants have time to size up before cold weather sets in.
Transplanting & Spacing
Space plants about 8 inches apart in rows 18 inches apart. If direct-seeding, sow ¼ to ½ inch deep and thin to the strongest seedlings.
Care
Kale tolerates a wide range of conditions and is genuinely one of the easier crops in the brassica family. Keep soil consistently moist — drought stress toughens the leaves and makes them more bitter. Watch for cabbage worms, aphids, and flea beetles; row cover helps while plants are getting established. Kale's cold hardiness is one of its best features: young plants shrug off temperatures down to 25°F, mature plants tolerate far colder conditions, and the flavor actually improves after a frost.
Harvest
Kale is a classic cut-and-come-again crop. Harvest outer leaves as needed and let the center keep producing — plants will continue putting out new leaves well past the first fall frosts.
Growing Kale for Seed
The Late-Summer Start
Growing kale for seed calls for different timing than growing it for the table. Plant in July to early August so plants reach a moderate, established size — solid vegetative growth, not full maturity — before hard frost hits. That's the sweet spot: big enough to survive winter, not so large that it becomes prone to rot under winter protection.
This is a two-season project. A plant started in late summer of year one gives you a seed harvest in year two, after it flowers the following spring.
Why the Cold Matters
Kale is a true biennial: leaves the first year, a flower stalk the second (or occasionally late in the first year, if a young plant hits an unseasonable cold snap). That winter cold period is what triggers the switch from leafy growth to flowering. Minnesota winters provide far more cold than kale actually needs for this — the real job here is keeping the plant alive through it, not adding more chill.
How Many Plants?
Kale is insect-pollinated and outbreeding, so it needs company to set good seed:
Bare minimum: 5 plants
Maintaining a variety over the years: 20–50 plants
Preserving a rare heirloom: 80+ plants
Home gardens can scale down from the top end, but the rarer the variety, the more it benefits from a bigger population.
What to Select For
Walk your overwintered plants before they bolt in spring and choose your seed parents then — winter survival, leaf type, color, texture, and bolt timing. Once a plant sends up a flower stalk, you've lost the chance to select against it, so all the culling has to happen beforehand.
Quick tip: If a spring-planted kale bolts unexpectedly the same season after an early cold snap, most seed savers skip saving that seed. Saving it just selects for a tendency to bolt too soon — not a trait worth passing along.
Isolation: Distance, Timing, and a Species Shortcut
By Distance
Kale needs at least 800 feet to ½ mile of isolation from anything else flowering in the same species group — whether that's another kale variety or a different oleracea crop entirely.
By Timing
Not every brassica flowers on the same schedule, and that's useful. An overwintered kale planted the year before typically finishes flowering by early-to-mid summer, while a spring-planted broccoli (also Brassica oleracea) might not start flowering until much later. Coordinating who blooms when is a real, if imperfect, isolation tool for gardens without much room to spare.
By Species (Kale's Built-In Advantage)
Here's the twist: Russian and Siberian kale aren't the same species as most other kale. They're Brassica napus, while curly, Lacinato, and Scotch kale are Brassica oleracea — the same species as broccoli, cabbage, and the rest of the cole crops. That means you can flower an oleracea kale and a Siberian kale side by side in the same season with zero crossing risk between the two, even though they both go by "kale." Each still needs its own isolation from other members of its own species.
What Crosses With What
Brassica oleracea kale (curly, Lacinato/dinosaur, Scotch types) crosses with:
Broccoli
Cabbage
Brussels sprouts
Cauliflower
Kohlrabi
Collards
Gai lan (Chinese broccoli)
Ornamental kale
Brassica napus kale (Russian, Siberian types) crosses with:
Rutabaga
Canola / oilseed rape
Other Siberian/napus kale varieties
Neither type crosses with turnips, Chinese cabbage, or mizuna (Brassica rapa), or with brown mustards like Ruby Streaks (Brassica juncea) — different species entirely.
The one fact worth remembering: Russian/Siberian kale won't cross with regular kale, broccoli, or cabbage — but it will cross with rutabaga. If you're trying to grow several brassicas for seed in a small garden, pairing one oleracea crop with one napus crop is a free isolation strategy.
Overwintering in Minnesota (Zone 5a)
Kale is about as cold-hardy as garden vegetables get. Young plants shrug off temperatures down to 25°F without real damage, and mature plants handle far colder conditions outright — with the flavor actually getting better after a frost. Several varieties are rated winter-hardy to Zone 6 on their own, and Minnesota gardeners routinely push kale further than that with a little help.
Protecting Plants in the Ground
Heavy mulch — pile straw or leaves around the base once hard freezes set in, insulating the crown and roots
Cold frame — the most reliable option for a smaller seed-saving planting, and a real boost to winter survival odds
Low tunnel — row cover or plastic over hoops, a middle ground between open mulching and a full cold frame
Spring Emergence
Pull back heavy mulch gradually as the weather warms to prevent rot and mold from building up around the crown. When plants bolt and send up a flower stalk, that's the normal, expected start of the second-year seed cycle — not a problem. Keep watering through flowering and seed-fill; drought stress at this stage cuts into seed yield.
Good news for beginners: Kale is one of the more forgiving Brassica oleracea crops to overwinter in place here. Its natural cold tolerance does a lot of the work that crops like cauliflower or cabbage need a root cellar for.
Harvest & Processing Seed
Let seed pods dry brown and papery right on the plant — this takes several weeks after flowering wraps up. Harvest once pods are dry but before they shatter; for a larger planting, cut whole seed stalks and finish drying them undercover so you don't lose seed to shattering in the garden.
Thresh by hand-rubbing pods for small batches, or flailing for bigger harvests. Winnow with screens sized just smaller than the seed, then fan away the fine chaff. Let seed fully dry down before storage .
Storage
Store cleaned seed cool, dry, and dark. Kale seed stays viable for about 4 to 5 years under good storage conditions. A quick germination test before adding it to the catalog is always worth the extra week.
Sources
University of Minnesota Extension, Growing Collards and Kale in Home Gardens — Minnesota planting timing and the species distinction between kale/collards (Brassica oleracea) and Russian/Siberian kale (Brassica napus var. pabularia). extension.umn.edu
Seed Savers Exchange, Seed Saving Guide — isolation distances, population sizes, and seed viability for both oleracea and napus kale types. shop.seedsavers.org
Annapolis Seeds / Heartseed Farm, Growing and Seed Saving: Kale — isolation-by-timing strategy based on flowering periods. annapolisseeds.com
Redwood Seeds, Brassica Basics: Seed Saving 101 — species-level crossing examples and guidance on discarding accidentally early-bolted seed. redwoodseeds.net
Insteading, How to Save Brassica oleracea Seeds — isolation strategies for small home gardens. insteading.com
The Old Farmer's Almanac, Kale: Planting, Growing, and Harvesting — cold tolerance thresholds for young and mature kale. almanac.com
Curious about growing kale for seed this year, or joining the Seed Garden Guild? Reach out to the Como Community Seed Library — we'd love to help you get started.
