Biennials such as carrot, beet, cabbage, kale, leek and onion grow leaves and roots in their first year and flower and seed in their second after a cold period, so to harvest seed you must keep a healthy root through winter and plant it out in spring. The same cold that makes them flower can make a small young plant bolt in its first year if it is exposed to prolonged cold, so size and timing of sowing matter. Keep seed plants of different varieties apart, because many biennials cross-pollinate.
A biennial is a plant that completes its life cycle in two growing seasons, growing vegetatively in the first and flowering and setting seed in the second, usually after a winter cold period.
Winter Cold Is The Signal To Flower, And Too Much Too Early Causes Bolting
Many biennial vegetables will not flower until they have been through a spell of cold. This requirement is called vernalisation. A plant that is large enough when the cold comes will flower the next spring; a very young plant that meets a long cold spell may flower early and waste the crop. How large is large enough, and how much cold, varies by species and variety.
What You Need
- Seed-quality plants — healthy, true to type, from the best of the crop (Seed Selection).
- A place to hold roots — in the ground under thick mulch, in a clamp, or in a cellar (Root Cellars).
- Space and isolation for the seed crop (Seed Saving gives isolation distances).
Method
Step 1: Select the seed plants in autumn. Keep the best roots, heads or bulbs. Do not eat all of the best and seed from the rest; the next generation comes from whatever you keep.
Step 2: Hold them through winter. In mild climates leave the plants in the ground with deep mulch. Where the ground freezes hard, lift roots and store them cool and damp in the cellar, then replant in spring. Brassicas such as cabbage are often lifted whole and stored with their roots.
Step 3: Replant in early spring as soon as the ground can be worked, at the spacing for a seed crop (wider than for eating).
Step 4: Support and protect the flower stalks, which can be tall. Stake against wind.
Step 5: Isolate. Biennials such as carrot and beet cross with wild or neighbouring relatives. See Seed Saving for isolation distances.
Step 6: Harvest seed when dry and store it cool, dry and labelled (Community Seed Bank Management).
Diagnosing Problems
- Plants bolting in their first year — sown too early, exposed to cold when small; sow later.
- Roots rotting in storage — stored too warm or wet, or damaged when lifted.
- No flowering in spring — the plants never had enough cold, or never grew large enough.
Common Mistakes
- Eating the best roots and seeding from the rest — it breeds the crop downhill.
- Overwintering too few plants — losses and poor pollination reduce the seed crop.
- Forgetting that onion family bulbs may flower after a cold spell in their first year — plant sets and seed carefully.
What’s Next
- Learn next: Spring-Started Transplants — the other side of the spring calendar
- Combine with: Root Cellar Integration — holding the roots