Many people don't seem to realize that temperature can be a critical factor regarding whether theirseed starting enterprise eventually is successful or a failure. Too low, or too high, of a temperaturecan have a dramatic effect on what your plants ultimately become. Additionally, plants have differingheat requirements at various stages in their growth - knowing this can greatly improve your results.
Seeds
Many seeds, even for cool-season vegetables like lettuce or broccoli, will germinate much betterin warm conditions - at least to a degree (no pun intended). Some heat-loving varieties won't evencome up if the soil is too cool. But heat can be overdone. For example, lettuce seed will often godormant if the soil temperatures stay above 85F - a condition known as thermodormancy. Allseeds have an optimal temperature for germination, above which their failure rate increasesmarkedly. Good seed companies, such asTerritorial Seed andJohnny's Selected Seeds, will listthe best temperature range for germination of each vegetable. One of the things I really like aboutJohnny's catalog is they have a graph printed with each variety, diagramming the germination rateover 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 goodresults with an Electra-Grow mat, which is available from many seed companies for roughly $50. Thesmaller-size mat is just the right size for one of my 11"x22" seed-starting trays, and it sitsnicely on the shelf under my grow lights. You can leave it plugged in all the time, or have it cycleoff at night as I do now. Some seeds require a warm/cool temperature cycle, but even those that don'tseem 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-stylerefrigerators (new ones don't always vent heat at the top). I've known some gardeners who placetheir seed containers by the wood stove; if you try this, be very careful not to overheat thepots! With any of these locations, you have to watch the seeds carefully. When germinationbegins, move them to a well-lit location.
It is important to remember that some seeds actually prefer cooler temperatures. This isespecially true of certain flowers and perennials. When germinating ornamental plants, you mightwant to refer to the excellentSeed GerminationDatabase put together by Tom Clothier and Asle Serigstad.
Seedlings
Seedlings almost always have a lower heat requirement than seeds. They will growmuch better if left on the mat, of course; but this lush growth will fare very poorlywhen the plants are moved out into your garden, even in summer. Many gardeners raise theirplants in warm settled conditions, then harden them off for a few days before moving themto the garden. Hardening off involves gradually exposing your plants to outside conditions, firstfor a little while, then gradually (over a week or so) lengthening their time outside. New cellsgrown during this hardening off period will be tougher than the earlier growth, but that softergrowth 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 willnot set them back. One advantage to this is that all the growth is "hard", which means no hardeningoff 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 cellsalso mean the plant is more resistant to diseases and pests. While these plants grow slower atfirst - and frankly don't look as lush and wonderful as warm-grown plants - they race out of thestarting 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 anexample: I start my seed around March 1. When they start germinating, I pull them off theheat 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 dayI put them outdoors, but bring them in by early evening. Under these conditions the plants growslowly - by late April they are still only about a foot tall. But they are tough, and I can putthem out under my PVC hoophouse before May 1 most years, eventhough they'll almost certainly be exposed to temperatures in the 30s a few times. Growing thisway, 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 temperaturescan 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 40smay not die, but they probably will never amount to much either. Peppers, eggplant, and melonsare even more tender. If you want to try this method, I strongly recommend you look at myVegetable Garden Timetable first, and remember it'sbetter 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 adramatic 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 makessense - after all, with a mature plant it's not as important to be able to grow more, all it needsto do is produce viable seed. This means that adaptations that would not really work for a seedlingcan 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. Undera cloche a plant will usually grow better than out in the open, simplybecause the dew or frost is kept off of it; even if the air in the cloche gets as cold as theoutside 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 droppedinto the mid-30s. The only plant that was under a sagging part of the cloche had damage that wasobviously caused by cold condensation dripping off the plastic.
All contents © Travis Saling
This page was last updatedNovember 18, 2013
