Growing Primula from Seed: Chilling, Surface Sowing and Transplanting seedlings

Fresh seed and a cold spell

Primula seed loses viability fast. Seed collected and sown the same season germinates far better than seed kept warm and dry in a drawer for a year, and packets bought late in the season are often disappointing for exactly this reason. If you have to store it, keep it cold and dry in a sealed container in a refrigerator, not in a warm cupboard.

Many primulas also need a cold period before they will germinate, which is why trays sown in a warm greenhouse and kept at 70F (21C) sit for weeks and produce nothing. The simplest method is to sow in autumn or late winter and stand the tray outdoors in a shaded, sheltered place where it gets natural cold, protected from heavy rain and from mice. Germination then happens on its own as temperatures rise.

The controlled alternative is to sow, water, seal the tray in a plastic bag and put it in a refrigerator at 34F to 40F (1C to 4C) for three to four weeks, then bring it out to a cool bright place at 50F to 60F (10C to 16C). That cool bright stage is essential; warmth after chilling does not speed germination up, it suppresses it. Above about 68F (20C) many primula seeds simply refuse.

Sow on the surface and never cover the seed

Primula seed needs light to germinate. Covering it with compost, even lightly, is the commonest reason a sowing fails outright, and it fails silently because the seed is fine enough that you cannot see whether it is buried or not. Sow on the surface and leave it there.

Fill a clean tray or pot with a fine seed-starting mix, firm it lightly and level it, then water it thoroughly from below by standing it in a shallow tray of water until the surface darkens. Sowing onto compost that is already wet means you never have to water from above and wash the seed into the compost. Scatter the seed thinly across the surface, press it down gently with a flat piece of wood so it sits in contact with the compost, and do not cover it.

A sheet of glass, a clear lid or a loose plastic bag over the top holds humidity so the surface does not dry, which matters enormously when the seed is sitting on top of it. Keep the tray in bright indirect light rather than direct sun, which will cook it under the cover, and ventilate for a few minutes daily to keep the air from going stale.

The seedlings are tiny and slow, and that is normal

Germination is uneven. The first seedlings may appear in two weeks and the last in six, so do not throw a tray away because it looks half empty; keep it cool and damp and it will keep filling. The seedlings that come up are minute, a pair of round seed leaves barely larger than a pinhead, and they can sit at that size for two or three weeks before the first true leaf appears. Nothing is wrong.

Damping off is the main danger through this stage. It shows as seedlings collapsing at soil level in a spreading patch, and it comes from stale wet air and overcrowding rather than from anything you can spray away. Use clean trays and fresh compost, sow thinly, take the cover off as soon as the first seedlings appear, and water from below rather than over the top. Air moving across the surface of the compost does more to prevent it than anything else.

Keep them cool. A tray of primula seedlings on a warm windowsill stretches into pale, drawn, floppy growth that never makes a decent plant, and it dries out at the surface where the roots still are. A cool bright place at 50F to 60F (10C to 16C) grows short, dark green, tough seedlings that transplant well.

Write down the sowing date, the date the cold treatment ended and the date the first seedling appeared. Primula germination varies so much between batches of seed that your own record for your own conditions is the only reliable guide to whether a tray is late or has failed.

Transplanting seedlings and growing on to flowering size

Transplant when each seedling has two true leaves beyond the seed leaves, which is usually four to six weeks after germination. Earlier than that the roots tear; much later and the seedlings are matted together and you lose more than you save. Work in a cool shaded place so the exposed roots are not drying while you handle them.

Lift each seedling by a leaf, never by the stem, using a plant label or a pencil to lever the roots free from underneath. Set it into a cell or small pot of moist potting mix at the same depth it was growing, with the tiny crown level with the surface, and firm gently with the tool rather than your finger. Water them in from below and keep them shaded for the first two or three days.

Grow them on cool. Once frost is past they are far happier outdoors in shade than under glass, where summer heat stalls them. Feed at quarter strength every two weeks, pot up before the roots circle the cell, and plant into their permanent shaded positions in early autumn so they root in before winter. Expect flowers the following spring, a full year from sowing, and a much better plant in the year after that.

1. Wait for two true leaves The seed leaves are round and plain; the true leaves that follow have the crinkled texture of the adult plant. Two of those, and the seedling has enough root to move.
2. Lift by a leaf, never the stem Lever from underneath with a label or pencil and hold a seed leaf between finger and thumb. A crushed stem kills the seedling; a torn leaf does not matter at all.
3. Set at the same depth and water from below Plant so the crown sits level with the compost surface, not buried, and stand the tray in shallow water until the surface darkens. Shade for two or three days while they recover.
4. Move them outdoors into shade After the last frost, harden them off over a week and stand them in a shaded, sheltered spot. They grow better in cool outdoor air than in any greenhouse through summer.