Starting Seeds in Hot Weather

Season extension and year-round vegetable production include gardening in hot weather, when there are some particular challenges to overcome. Some seeds are hard to germinate when the weather is hot. The first tool is information on normal germination temperatures for various crops, and ways to achieve the temperature goals. Other techniques include soaking and pre-sprouting seeds.

Reader Contribution by Pam Dawling
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by AdobeStock/Anna Jurkovska

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.

Summer temperatures can make it hard to establish crops which will have no difficulty growing once the weather cools down. Notice that if you can lower soil temperatures to get germination, in most cases you only have to do it for a few days. Getting good soil contact is important, so tamp the row well after planting.

Soaking and Pre-Sprouting Seeds

Some seeds benefit from soaking before sowing. The soaking time depends on the seed’s size: bigger seeds benefit from longer soaking than smaller seeds. We generally soak beans and peas overnight, which helps the large seeds get all the water they need to absorb for the initial sprouting. After that the smaller amounts needed to keep growing are more easily found. Don’t soak legumes so long that the seed coat splits, as they then lose vital nutrients and may become vulnerable to attack by fungi.

Small seeds that have been soaked tend to clump together, so after draining off as much water as possible, mix them with a dry material like uncooked corn grits, oatmeal or bran, or use coffee grounds or sand. If you plan to put soaked or sprouted seeds in a seeder, dry off their surfaces by spreading them out in a tray for a while. Experiment on a small scale ahead of a big planting, to make sure your seeder doesn’t just turn the seeds to mush, or snap off any little sprouts.

Beets are notorious for spotty germination — their seed coats contain a germination inhibitor. Presoaking beet seed for two hours can help dissolve this compound. Room temperature water is better than cold water, and running water is the best, I’ve heard. I suspect when I’ve had failures with soaked beet seed it’s because I soaked them for too long and they suffocated due to a shortage of oxygen. Another option is to pre-sprout them just until small red shoots are seen.

To pre-sprout seeds for extended-season growing, first soak them. Then drain off the water which has not been absorbed, and put the seeds in a suitably cool place. Rinse twice a day, draining off the water. Special plastic draining lids are sold for mason jars, for people who grow sprouts to eat. These are great to use, but you can also make your own with a piece of nylon window screen held on with the lid ring or a rubber band. For large quantities of seed we use plastic jars from catering sizes of mayonnaise and mustard. A pasta strainer is a helpful tool, as is a sieve held upside down closely in the mouth of the jar.

Usually it’s best to sprout the seed just until you see it has germinated. Seeds with long sprouts are hard to plant without snapping off the shoot. For most crops, 0.2 inch (5 mm) is enough. For lettuce half that length is good, and one day may be time enough. If your pre-sprouting has got ahead of the weather or the soil conditions, slow down growth by putting the seed in the refrigerator.

Pre-sprouting spinach seeds in late summer is very worthwhile. For this we do the whole sprouting process in the fridge, and I confess we don’t rinse them at all! One week is a good length of time for fridge sprouting of spinach. Give the jar a quarter-turn each day to tumble the seeds and even out the moisture.

If you have leftover soaked or pre-sprouted seeds, you can store them in the fridge for a while until you see if you got good germination. If not, and the seeds still look good, go ahead and fill the gaps in the row. Leftover soaked pea seed can grow a crop of pea shoots for salad.

In a future post I will provide 20 Tips for Success in Germinating Seeds in Hot Weather.  This information is an extract from The Year-Round Hoophouse © Pam Dawling and New Society Publishers.

Pam Dawling has worked at Twin Oaks Community in central Virginia for more than 27 years, growing vegetables for 100 people on 3.5 acres and training many members in sustainable vegetable production. She is the author of Sustainable Market Farming and The Year-Round Hoophouse. Pam often presents workshops at MOTHER EARTH NEWS FAIR and at sustainable agriculture conferences. She is a contributing editor with Growing for Market magazine, and a weekly blogger on SustainableMarketFarming.com. Read all of Pam’s MOTHER EARTH NEWS posts here.


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