Seed Starting Guide

Starting from seed is cheap, and it is the only way to grow varieties that never reach a garden centre bench. It is also where most people lose plants, almost always to four causes: sowing too early, too little light, too much water, and rushing the move outdoors.

What You Actually Need to Buy

• Modular trays. Modules of roughly 4-5 cm (1.5-2 in) square suit most crops. Avoid open trays unless you enjoy untangling roots.
• Seed compost, not garden soil. Fine, low-nutrient and free-draining. Garden soil compacts and carries fungal spores.
• A light source. A cheap LED shop light hung 5-10 cm (2-4 in) above the leaves, on a timer for 14-16 hours a day.
• A solid tray to bottom-water in.
• Labels and a pencil. You will not remember which row was the sweet pepper.

A heat mat is worth adding if your sowing area sits below 18C (65F), as is a small clip fan.

A windowsill is not a lighting plan: in late winter it gives a fraction of open-sky light, from one side, which is why windowsill seedlings lean.

Timing: Counting Back From Your Last Frost

Everything here is measured backwards from your average last frost date. Find that date first; reading your frost dates covers how.

Then subtract. A tomato wants 6-8 weeks indoors, so with a last frost around 15 May you sow in late March. A courgette wants 2-3 weeks, so it goes in during late April, not in March alongside the tomatoes.

The most common mistake by a wide margin is sowing too early. A seedling that has sat in a small module for eleven weeks is root-bound, hungry and stressed, and one sown a month later will overtake it in the ground. Late is recoverable; early is not.

Sowing Depth, Spacing and Mix

The old rule holds: sow to roughly twice the seed's width. A few crops, lettuce and celery among them, need light and should sit on the surface pressed into firm contact.

Moisten the compost before you fill, not after; aim for the texture of a wrung-out sponge. Fill modules loosely, tap the tray down once, level off. Do not compress with your thumb, which turns a fine mix into an airless block.

Sow two seeds per module for anything expensive or unreliable, one for large-seeded crops. When both come up, cut the weaker one at soil level with scissors; pulling it tears the root of the keeper.

Germination: Warmth First, Light Second

Until a seed breaks the surface it does not care about light, only moisture and temperature. Peppers at 18C (65F) may take three weeks and come up unevenly; at 27C (80F) the same seed is up in six or seven days. That is the best argument for a heat mat.

Cover the tray to hold humidity, and move it off heat and under lights the moment the first seedlings appear. Leaving a germinated tray warm, dark and covered for even two extra days produces pale, elongated stems that never recover. If seedlings lean or stretch, the light is too far away, too weak, or not on long enough.

The Week That Kills Most Seedlings

Days five to fourteen is when damping off arrives: a seedling topples with a thin, water-soaked stem pinched at soil level, and by morning three neighbours have gone the same way. It is a group of soil fungi that thrive in still, wet conditions.

Prevention is the whole treatment, because there is no cure once a stem has collapsed.

• Water from below, and let the surface dry between waterings.
• Remove the humidity cover as soon as germination is under way.
• Run a small fan on low for a few hours a day. It also thickens stems.
• Use fresh compost and clean trays; a hot soapy wash is enough.
• Do not fertilise seedlings still on their seed leaves.

Potting On

Once a seedling has its first pair of true leaves and roots show at the drainage holes, it needs more room. Move it to a 7-9 cm (3-3.5 in) pot of normal potting mix, which unlike seed compost contains nutrients.

Handle seedlings by a leaf, never the stem: a crushed leaf is replaceable, a crushed stem is not. Tomatoes can be buried to their seed leaves and will root along the stem; most other crops go in at the depth they were growing. From here, feed at half strength every ten days.

Hardening Off Without Losing the Tray

Indoor seedlings have thin leaf cuticles and no wind conditioning. Put them straight out and you get bleached, papery leaves within a day.

A workable seven-day schedule, once nights are reliably above about 10C (50F) for tender crops:

1. Days 1-2: two hours in full shade, sheltered from wind.
2. Days 3-4: four hours, with an hour of it in morning sun.
3. Days 5-6: most of the day out, half in sun; bring in at night.
4. Day 7: leave out overnight if the forecast is mild.

Skip a day if the weather turns, and water more often than feels necessary; small pots outdoors dry fast, the same trap described in watering schedule mistakes.

Transplanting and Aftercare

Plant out on a still, overcast afternoon or in the evening, never at midday. Water the pots an hour beforehand so the root ball comes out intact. Dig a hole slightly wider than the ball, firm gently, then water in with 1-2 litres per plant. Mulch afterwards, keeping it clear of the stem.

Expect three to seven days where nothing visibly grows. That is normal root establishment, and the answer is not fertiliser. If slugs are a problem, protect the first fortnight with collars or evening patrols; a seedling that loses its growing tip is finished.

A written sowing calendar is what turns one good year into a repeatable one. If you would rather not keep that spreadsheet by hand, Vegarden can work the dates back from your own frost window and tell you when each tray is due.

Troubleshooting Common Failures

Nothing germinated. Usually temperature or old seed. Check the packet date — parsnip and onion seed is effectively dead after a year — and whether the surface dried out on day three.

Long, floppy, pale stems. Not enough light, or too warm after germination. Add light hours, lower the fixture, drop the temperature a few degrees.

Stalled at two true leaves. Root-bound or hungry. Pot on and start a half-strength feed.

Purple undersides. Usually cold locking up phosphorus rather than a real shortage. Warm them and it resolves.

White fuzz on the compost. Harmless surface mould, but a signal you are overwatering. Scrape it off, add a fan, water less.