How to choose the right amount of seeds
The short answer: always sow about 20% more seeds than the plants you need — germination is never 100%, and the rate printed on the packet drops every year the seed is stored. Seed health and viability matter most: check the packing year and minimum germination percentage on the packet, and give seeds the five conditions below.
What do seeds need to germinate?
Water — enough moisture around the seed to soften it and start respiration; germination begins the moment respiration does.
Air — seeds need oxygen and some carbon dioxide. Loose, moist soil provides both; overwatering blocks the soil pores and suffocates them.
Temperature — see the table below; a heating mat under selected trays creates ideal conditions when you can't heat the whole greenhouse.
Light — depending on species, light can stimulate or inhibit germination. Most vegetables behave similarly, but check the packet for specifics.
Soil — germination soil should hold water well yet drain effectively, and be free of pests, diseases and weeds. Mix your own or buy it ready-made.
| Crop type | Examples | Best germination temperature |
|---|---|---|
| Cool-season | Lettuce, broccoli | 10–24 °C |
| Warm-season | Peppers, tomatoes, eggplants | 24–32 °C |
How many seeds should you sow for the yield you want?
Start from the desired yield per vegetable and work forward. Expected yield per plant is listed in some seed catalogues, sometimes on the packet, or — best of all — in your own harvest journal from previous seasons.
Tomato seedlings already germinated.
Worked example: 45 kg of tomatoes
| Step | Result |
|---|---|
| Desired yield ÷ yield per plant | 45 kg ÷ ~2.25 kg = 20 plants |
| +10–15% plants (weather, moisture risk) | ~23 plants |
| +20% seeds (germination losses) | ~30 seeds to sow |
Thirty seeds gives a comfortable buffer — in a good year those plants can produce up to 56 kg. Surplus seedlings can be sold or given away, and knowing the exact plant count also lets you calculate the greenhouse area you'll need.
Frequently Asked Questions
Do old seeds still germinate?
Often, but at a lower rate — viability falls every year of storage. Check the packing year on the packet and sow extra to compensate.
Why aren't my seeds germinating in moist soil?
Usually temperature (warm-season crops need 24–32 °C) or waterlogged soil blocking oxygen. A heating mat plus lighter watering fixes most cases.
How does automated watering help at the seedling stage?
Seedlings need small, even doses — exactly what timers get wrong. Moisture-based zone control keeps trays damp without flooding the pores seeds breathe through.