Many people don't seem to realize that temperature can be a critical factor regarding whether their seed starting enterprise eventually is successful or a failure. Too low, or too high, of a temperature can have a dramatic effect on what your plants ultimately become. Additionally, plants have differing heat requirements at various stages in their growth - knowing this can greatly improve your results.


Many seeds, even for cool-season vegetables like lettuce or broccoli, will germinate much better in warm conditions - at least to a degree (no pun intended). Some heat-loving varieties won't even come up if the soil is too cool. But heat can be overdone. For example, lettuce seed will often go dormant if the soil temperatures stay above 85F - a condition known as thermodormancy. All seeds have an optimal temperature for germination, above which their failure rate increases markedly. Good seed companies, such as Territorial Seed and Johnny's Selected Seeds, will list the best temperature range for germination of each vegetable. One of the things I really like about Johnny's catalog is they have a graph printed with each variety, diagramming the germination rate over an optimal temperature range.

Since soil temperature can be so critical to germination success, many gardeners use heat mats. There are all sorts of different kinds that range all over the cost continuum. I have had good results with an Electra-Grow mat, which is available from many seed companies for roughly $50. The smaller-size mat is just the right size for one of my 11"x22" seed-starting trays, and it sits nicely on the shelf under my grow lights. You can leave it plugged in all the time, or have it cycle off at night as I do now. Some seeds require a warm/cool temperature cycle, but even those that don't seem to germinate just as quickly in this situation as they do if the mat is always left on.

Of course there are other options - warm places around your house that are good for seed-starting. Some people set their seed flats on their water heater, while others use the top of old-style refrigerators (new ones don't always vent heat at the top). I've known some gardeners who place their seed containers by the wood stove; if you try this, be very careful not to overheat the pots! With any of these locations, you have to watch the seeds carefully. When germination begins, move them to a well-lit location.

It is important to remember that some seeds actually prefer cooler temperatures. This is especially true of certain flowers and perennials. When germinating ornamental plants, you might want to refer to the excellent Seed Germination Database put together by Tom Clothier and Asle Serigstad.


Seedlings almost always have a lower heat requirement than seeds. They will grow much better if left on the mat, of course; but this lush growth will fare very poorly when the plants are moved out into your garden, even in summer. Many gardeners raise their plants in warm settled conditions, then harden them off for a few days before moving them to the garden. Hardening off involves gradually exposing your plants to outside conditions, first for a little while, then gradually (over a week or so) lengthening their time outside. New cells grown during this hardening off period will be tougher than the earlier growth, but that softer growth is still there inside the plant.

In my opinion, though, it's better to raise the plants the whole time in as cool conditions as will not set them back. One advantage to this is that all the growth is "hard", which means no hardening off is required. It just makes sense to grow them in a similar climate to what they'll face outside; after all, even in July our average low temperature is only in the low-to-mid 50s. Tougher cells also mean the plant is more resistant to diseases and pests. While these plants grow slower at first - and frankly don't look as lush and wonderful as warm-grown plants - they race out of the starting blocks as soon as they are set outdoors. These stronger plants yield sooner than larger, softer-grown ones, even if those are hardened off correctly.

To accomplish this I've set up my growing area on our unheated back porch. Using tomatoes for an example: I start my seed around March 1. When they start germinating, I pull them off the heat mat. At this time of year the temperature in the back porch drops to around 50F at night (sometimes lower), and gets up to the low 60s during the day. If we have a warm sunny spring day I put them outdoors, but bring them in by early evening. Under these conditions the plants grow slowly - by late April they are still only about a foot tall. But they are tough, and I can put them out under my PVC hoophouse before May 1 most years, even though they'll almost certainly be exposed to temperatures in the 30s a few times. Growing this way, I get a first harvest by late June or early July.

If you follow this method, you do have to be careful. Exposing heat-lovers to colder temperatures can shock them permanently. This means you can't start them too early - they can be grown hard, but only to a point. Tomato seedlings that are constantly hit with temperatures in the low 40s may not die, but they probably will never amount to much either. Peppers, eggplant, and melons are even more tender. If you want to try this method, I strongly recommend you look at my Vegetable Garden Timetable first, and remember it's better to err on the late side!

Moving Them to the Garden

Once plants get past the seedling stage they are usually more adaptable to colder temperatures. This is especially true when they get to the flowering and fruiting stage. Peppers provide a dramatic example. Even the toughest pepper seedlings can be damaged by temperatures below about 45F. In late fall, however, I've seen pepper plants survive light frost! This survival mechanism makes sense - after all, with a mature plant it's not as important to be able to grow more, all it needs to do is produce viable seed. This means that adaptations that would not really work for a seedling can be advantageous when present in an older plant.

It is also true that, all other things being equal, cold air does less damage than cold water. Under a cloche a plant will usually grow better than out in the open, simply because the dew or frost is kept off of it; even if the air in the cloche gets as cold as the outside air! This is basically a function of heat capacity - air holds much less heat than water does. Since your plant is mostly water, it takes quite a bit of cold air to drop the plant's temperature. In my garden I've seen this illustrated: Tomatoes, set under a cloche, survived a night that dropped into the mid-30s. The only plant that was under a sagging part of the cloche had damage that was obviously caused by cold condensation dripping off the plastic.

All contents © Travis Saling
This page was last updated November 18, 2013