Viability and vigor of seeds deteriorates when they are stored in warm places, especially if containers are not airtight, and the air is humid. If you have crops you grow in spring and again for the fall, store those seeds in a cool place over the summer.
Chilling lettuce seed can help germination in hot weather. We make a practice of putting our spinach seed in double ziplock bags and putting it in the freezer for two weeks before we attempt late summer sowings. This can trick the seed into germinating better.
Older lettuce seed suffers less from heat-induced dormancy. A few months of storage can make the difference, but you might consider buying seed of your favorite hot-weather varieties a year ahead.
Sowing Seeds Indoors
Consider transplanting, even for crops you normally direct seed.
Plants in plug flats, cell packs or soil blocks will transplant with less shock than plants from open flats.
If outdoor temperatures are impossible, start your seeds in your refrigerated storage or your house, either in flats. If it’s important to keep the space clean, put the flat in a clear plastic bag and open it a couple of times a day to let fresh oxygen in. The floors indoors are usually cooler than shelves.
Wet newspapers are another option for covering flats until seedlings emerge.
Sowing Seeds Outdoors
If soil temperatures are too high for good germination of your crop, look at options for cooling a small nursery bed with shade from other plants or shadecloth, boards or burlap bags.
Sowing at night (or at least as late in the evening as you can) will give better emergence because the temperature-sensitive phase of germination can occur while soil temperatures are lower.
Soils which tend to crust can make life difficult for small seeds that are slow to germinate. Reduce the chances of soil “capping” or crusting by keeping soil cultivation to a minimum, and watering ahead of planting. If crusting is likely, avoid tamping or rolling the soil hard. Some people sprinkle sand, grass clippings, straw or sawdust lightly along the rows.
Sowing Crops Challenged by Hot Weather
Getting enough water to the seed, and maintaining that level, can be tricky in hot weather. With carrots the challenge is to keep the soil surface damp until the seedlings emerge. For large-seeded crops (corn, beans) that we hand sow, we water the furrow generously before we plant. If we have a very well-watered furrow, we skip the tamping. Pre-watering ensures that the water is right there at seed level, where it is less likely to dry out. Often this is enough water to get the seedlings up out of the ground.
For lettuce, soil temperature must be lower than 80°F (27°C) - use a soil thermometer. Sow in the evening. After sowing (thickly), put ice (cubes or crushed ice) on top of the soil covering the seeds. Cover with shadecloth (50 percent shade is ideal), or tent screen windows, nylon window screen or sheer curtains. Use something air can flow through, to prevent overheating. Water with freshly drawn cool water at midday (possibly more than once a day) until the seed germinates.
Spinach is probably the crop that gives me the most challenging time. As noted earlier, it’s worth getting down as close as possible to 59°F (15°C) to get a good stand of healthy seedlings. Swiss chard germinates best at 85°F (30°C), so consider if chard or leaf beet could substitute for spinach if the fall is being impossibly hot.
Most people won’t be thinking about sowing onions in hot weather, but perhaps you want scallions to fill your CSA boxes, or a quick crop of cipollini mini-onions? Onions will still germinate at 90°F (32°C), but more slowly, and the percentage of normal seedlings will go down from over 90% to 73%.
Unlike flowers, most vegetables are indifferent to whether they have light or darkness when germinating. At high temperatures, celery and lettuce do better when they receive some light, therefore don’t sow too deeply. Seeds do need oxygen to germinate. Carrots germinate best in a high oxygen concentration. Cucurbits also are particularly sensitive to low oxygen/high CO2 - would using a fan help them germinate?
Chilled water, night watering and even ice on top of the rows can help reduce temperatures as well as supplying vital moisture.
Watering after sowing should be shallow and frequent, until the seedlings emerge. Make sure your direct-seeded crops don’t drown in an excess of water that pushes out all the air. After emergence, less frequent but deeper watering will encourage deeper roots, providing better heat tolerance.
Soaking Seeds, Pre-sprouting and Fluid Sowing
Revisit the information I provided in my, on soaking seeds
Simply pre-sprouting the seeds indoors, just until you see the beginnings of a shoot, can solve the problem of hot soils. Drain the sprouted seeds, and if they clump together, gently mix in some dry inert material like sand or uncooked corn grits, to make them easier to sow.
Fluid sowing is a novel way of helping seeds get established in difficult conditions. Sow pre-sprouted seeds in a jelly which protects them from damage. Emergence will be quick, which can be an advantage if you are working to extend the seasons to a maximum. Sprout the seeds, and once the shoots are 0.2" (5-mm) long, rinse and strain them gently.
Make up a starch or cellulose paste. A cup of thick cornstarch paste is easily cooked up by making a smooth slurry with 1–2 tablespoons (15–30 ml) of cornstarch and a small amount of cold water. Heat the rest of the cup of water, then stir in the slurry and boil for one minute to activate the cornstarch, stirring gently. If the paste is becoming too thick, stir in a little more water. Possible substitutes for cornstarch include the same quantity of xanthan gum, pectin, agar, gelatin, arrowroot, potato starch or tapioca. If using regular flour, use twice as much. Allow the paste to cool, then gently stir in the seeds. They should not sink when the mix is allowed to stand. If they do, the mixture wasn’t thick enough.
Start again and decant the seed layer to mix into the thicker paste. Pour the mixture into a plastic bag, and when you get to the bed, snip a small corner off the bag. Ideally the mixture should squeeze out into a damp furrow at about 1 tablespoon per foot of row (0.5 ml/cm), and should contain the number of seeds you want to plant in that distance. It may be worth experimenting on the kitchen counter first! After sowing, cover the seeds with soil in the usual way.
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 FAIRs 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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