Starting Seeds in Hot Weather



Use a (well-marked) soil thermometer to help you decide what can be sown. Photo by Bridget Aleshire. 

Season extension and year-round vegetable production include gardening in hot weather, when there are some particular challenges to overcome.  

Germination Temperatures

Some seeds are hard to germinate when the weather is hot. Sometimes the temperature is just too high for that seed, sometimes the soil dries out too fast. Some varieties of some crops have better germination at high temperatures than others. Consult the catalogs, especially ones from hotter parts of the country, and take a look at what grows in areas one or two zones warmer than yours. There are some techniques that can help, but the first tool is information: know the ideal germination conditions for your crop, the actual conditions, and the expected time to emergence under the conditions you’ve got.

There are excellent tables of germination temperatures in Nancy Bubel’s New Seed Starter’s Handbook and in Knott’s Handbook for Vegetable Growers. Nancy Bubel also has lists of the percentage of normal seedlings produced at different temperatures and flower seeds that need light to germinate, those that need darkness, and those that often do better with light. Get one of these books and a soil thermometer. This kind of information can save you from wasted effort. You may find surprises!

I knew that spinach does not germinate well at high temperatures. The tables say the optimum temperature range is 40°F–75°F (4°C–24°C) and the maximum temperature is 85°F (29°C). One year, after a frustrating time trying to germinate fall spinach, I took a closer look, which revealed that spinach will produce 82% normal seedlings at 59°F (15°C), but only 52% at 68°F (20°C), and a miserable 28% at 77°F (25°C). I hadn’t realized how worthwhile it is to somehow get lower temperatures for spinach, rather than working at the top of the possible range. Crops which germinate best at soil temperatures below 80°F (27°C) include beets, carrots, lettuce, onions, parsley, parsnips, peas and spinach.

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