Whoever wants the fattest tomatoes and the hottest chillies in summer starts as early as late winter on the windowsill. Because these warmth-lovers take so long to harvest that, sown outdoors, they would be too late. Pre-growing gives them the decisive weeks of head start.
That sounds like expert knowledge, but it is quite doable if you avoid the one commonest mistake. Most seed-raising fails not at germination but afterwards, when the seedlings go long and thin in the warm, dark room. How to raise sturdy young plants, we show step by step.
Why pre-grow at all?
Not every crop needs pre-growing. Beans, squash or radish you sow straight into the bed. But the slow warmth-lovers from warm climates need a long lead time that our spring outdoors does not offer.
Chief among them are tomato, pepper, chilli and aubergine. Sow them into the bed only after the last frosts and their fruit hardly ripens in time. Pre-grown on the windowsill, they stand as sturdy young plants at planting-out time and crop weeks earlier.
The biggest enemy: going leggy
No sooner have the seedlings germinated than the most important danger lurks. If they stand too warm and get too little light, they stretch desperately towards the window and grow long, thin and pale. This legginess makes the plant weak and prone to toppling.
How to raise sturdy young plants
Sow in seed compost
Use nutrient-poor seed compost, not fed potting soil. The lack of nutrients forces the roots to search vigorously. Sow the seeds only as deep as the packet says.
Germinate warm and moist
Keep the sowing evenly moist and warm; many crops germinate best at about 20 to 24 degrees. A clear hood or a mini greenhouse holds the moisture.
After germination, move bright and cooler
As soon as the seedlings are up, move them to the brightest window and lower the temperature. That is the decisive step against going leggy.
Prick out when the true leaves come
After the first two true leaves, set each seedling singly into its own pot with slightly richer soil. Hold the little plants only by the leaf, never by the tender stem.
Water sparingly and feed slowly
Water sparingly, best from below, so the roots grow deep. Only some weeks after pricking out do the plants need a little feed.
Warm to germinate, then bright and cool. Heed this switch and you raise sturdy young plants instead of long, pale floppy ones.
The core rule for pre-growing
Frequently asked questions
Why do my seedlings go so long and thin?
They go leggy because they stand too warm and get too little light. Move them at once to the brightest window and cooler, or give them a grow light. Already-leggy tomatoes you can set deeper when pricking out; they form new roots along the stem.
Do I really have to prick out?
With densely sown seedlings, yes, or they take light and space from each other. Whoever sows singly into small pots saves the pricking out. The advantage of pricking out is that the plants form a stronger root system.
Which soil do I use for seed-raising?
Special seed compost, which is nutrient-poor. In fed potting soil the seedlings form lazy, weak roots because the food is right there. After pricking out, the soil may then be a little richer.
Do I need a grow light?
Not essential, but it helps enormously. In the light-poor months of February and March, window light is often not enough and the seedlings go leggy. A simple grow light over the seedlings solves the problem reliably.
When may the young plants go outside?
Only after hardening off and after the last frosts in mid-May. Before that, the tender plants must be accustomed slowly to sun, wind and cold. How to do that is in Hardening off young plants.
