When should I sow my Tomato Seeds?

I ask myself this question every year. The seed packets say “sow in late winter” (when’s that?) or “late January to March” (more understandable) and, in different years, I’ve tried sowing from early January through until early April.

I think the answer depends upon a lot of variables some of which you can control and some of which you can’t. So lets look at them:

  1. The Weather
    The weather plays a part over which we have little or no control. Tomatoes originate from Mexico and grow best when the temperatures are always above 10C but below 35-40C. They also like light but not too much direct sunlight so juggling the seedlings/plants to give them the best conditions in the variety of external environments that we try to grow them in is a challenge. So knowing when the last frost is in your area and how much sunlight falls on your greenhouse whilst there are no leaves on the trees is something that you need to bear in mind and varies from yer to year.
  2. When/Where are you going to plant them?
    When and Where are related. Tomatoes suffer if their nighttime temperature stays below 10C for more than an hour or so and certainly don’t want a frost (they go purple, struggle and may not every recover). So you shouldn’t plant your tomatoes into their final place until after the last frost. Now when that is depends upon whether you’re planting them outside, in an unheated greenhouse or in a heated greenhouse and where in the world you live. (For me at home, the last frost is the end of May so planting outside has to wait until June, my allotment is on the other side of a north facing hill and the last frost is a few days later). An unheated greenhouse protects the plants and they can be planted a couple of weeks earlier. If you’re lucky enough to have a heated greenhouse temperature is not the issue.
  3. What are you going to do with them after they’ve germinated?
    Germinating tomato seeds is easy. Put them somewhere warm (20C) in decent compost and (depending on the age of the seeds) they’ll germinate in a week or two (the older the seeds the longer they’ll take but if they haven’t germinated in two or three weeks I think you can assume they’re not going to).
    However, once they’ve germinated, they need light and reasonable warmth to encourage them to grow strongly. I’ve found there are three steps:Seedlings:
    Seedlings are from when they first germinate until they’ve got their first true leaves. I sow my seeds in 7cm pots with about five seeds in a pot (perhaps more if they are old seeds which may not germinate). A seed tray will hold 15 pots and so 30 varieties (which is what I aim to grow) can be easily handled. Assuming the weather is reasonable (light & warmth), the seedlings can be carried out to the greenhouse every day and back in every night.

    Once the seedlings have their first true leaves, they should be separated into individual pots/plugs. At this point I carry out the first “cull” of my tomatoes, reducing the number of plants to about three/four of each variety. So now I have 90/120 plants. If they are in 7cm pots, this is 6-8 trays which is less convenient to move in and out of the greenhouse. However, I usually use plug trays which halve the number of trips.
    The plants will stay in the plugs for two – three weeks (its now five weeks since they were sown) and there will be nights when it will be warm enough for the tomatoes to stay out overnight in the unheated greenhouse.

    Small Plants
    Once the roots have filled the plugs, the plants need to be moved to larger pots (7-9cm) where they will stay until they are planted out. I cull them again, leaving myself with twice as many plants as I will need and, at this point, the plants need space & height, as well as warmth and light. However, by late April/early May the overnight temperatures in an unheated greenhouse are unlikely to fall below 10C for very long, so I tend to divide the plants, one set that will stay in the unheated greenhouse no matter what the nighttime forecast and the other half (30 plants) that will be brought in overnight if its going to be bad.

Tomato Seedlings 3 days after sowing


There’s no right answer, only what works for you. Your tomatoes will need about eight weeks from germination to planting out so seeds to be planted outside should be sown later than those in a cold greenhouse which in turn are later than those in a heated greenhouse.

How long you can look after the plants before they have to be planted out depends upon how much space you’ve got that can give the necessary warmth and light.

For me (in the middle of the UK and planting in unheated greenhouses) the answer that works seems to be early to mid March with a period of time molly-coddling the seedlings over night.

I’d be interested in your comments.