Should I sow seeds in modules or straight into the ground?

In a British spring the ground is usually the problem. Soil below about 7°C germinates almost nothing, and seed that sits in cold wet soil rots. Modules move germination somewhere warm and bright and put a robust plant into the ground rather than a vulnerable seed, which is why most experienced UK growers module-sow more than they used to.

Step by step

1. Sow direct where the root cannot be disturbed.Carrots, parsnips, radish, turnip, swede and beetroot form a taproot that forks or stunts if it is checked. Transplanted carrots are almost always deformed.
2. Sow direct where you need quantity.Salad rows, spinach, rocket, oriental leaves, and green manures. Raising two hundred lettuce in modules is a great deal of work for a crop that direct-sows easily in warm soil.
3. Use modules for everything that transplants.Brassicas, leeks, onions from seed, lettuce for planting out singly, peas, French and runner beans, courgettes, squash, sweetcorn, and virtually all bedding and hardy annual flowers.
4. Use modules to beat the calendar.Sowing in modules under cover in March gives a plant to put out in late April, weeks ahead of a direct sowing that has to wait for the soil to warm. On heavy or northern ground that difference is a whole crop.
5. Use modules to beat slugs.A 100mm plant with a woody base survives what kills a just-germinated seedling. On slug-heavy ground this alone justifies module-sowing beans, brassicas and squash.
6. Check the soil temperature before direct sowing.Most vegetable seed needs the soil above about 7°C, and beans, sweetcorn and squash want 12 to 15°C. A cheap soil thermometer pushed in at 50mm removes all the guesswork. Weeds germinating is the free version of the same test.
7. Warm the soil first if you must sow early.A sheet of clear polythene, a cloche or a cold frame over the bed for two or three weeks raises the temperature several degrees and dries the surface.
8. Sow module-raised plants at the right module size.Small cells for lettuce and brassicas, deeper cells for peas and beans, and root trainers or toilet roll tubes for anything with a long root that hates disturbance, like sweet peas and runner beans.
9. Do not let modules become pot-bound.The advantage evaporates if the plant sits in the cell too long. Roots at the base of the cell means plant it out or pot it on, not wait for better weather.
10. Harden off anything raised under cover.A plant that has never met wind or cold nights will check badly or die when planted out. A week in a cold frame or a sheltered spot with increasing exposure is not optional.

Tips

• Weeds germinating in a bed is the most reliable free soil thermometer there is.
• Sowing two or three seeds per cell and removing the weakest is more reliable than sowing one and hoping.
• Direct-sown carrots plus module-raised everything else covers almost every UK vegetable garden.

Common mistakes

• Transplanting carrots and parsnips — The taproot is the crop, and any check to it produces forking and stunting. These are direct-sow only.
• Direct sowing in March because the packet says March — Packets are written for the whole country and a good year. Cold wet soil rots seed. Wait for the soil, or sow in modules.
• Leaving module plants too long — A pot-bound module plant checks badly on planting out and often never catches up with something sown later straight into warm ground.

Alternatives

• Direct sowing: No equipment, no transplanting, no root disturbance. Vulnerable to cold soil, slugs, birds and drying out. Essential for root crops.
• Modules under cover: Warm reliable germination, a head start of several weeks, and a plant robust enough to survive slugs. Needs bench space, watering and hardening off.
• Root trainers or tubes: Deep narrow cells for peas, beans and sweet peas, planted out tube and all, so the long root is never disturbed.
• Sowing under a cloche in situ: A halfway house — direct sown, but with the soil warmed and the seedlings protected. Good for early carrots and salads.

Questions people ask