Seed-Starting in Hot Weather

Starting seeds in summer for a fall garden.

July 28, 2019
Seedlings in Pots

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Midsummer, and yikes, is it ever hot! It is time for me to be starting new plants for the fall from seed but conditions are just too hot. What to do? Here are a few tricks.

The seeds of different vegetables have optimum temperatures for germination and right now most of them will not like this heat.

  • Beets and carrots aren’t too bothered by hot soil. Their germination doesn’t start to drop off until the soil gets to be over 90 degrees.
  • However, the optimum temperature for lettuce is around 65 degrees and germination drops off rapidly after the soil heats up to the mid seventies.
  • Spinach does best at 70 degrees but by the time the soil is in the mid-80’s, forget about it.
  • Some brassicas don’t mind the heat. Cabbage, Chinese cabbage, and kale will sprout fine in soil as hot as 90 degrees, while broccoli prefers cooler soil, its optimum being around 75.
  • The germination on cauliflower drops off rapidly after soil temperatures hit 85.

A few Tricks for Seed-Starting Success

To increase your chances of success starting seeds in midsummer there are a few tricks:

  • Since soil heats up fast and dries out quickly in the sun, spread a light layer of straw over the seedbed to give it a bit of shade and to help keep the soil moist.
  • Plant seeds a little deeper than usual. The soil will be a touch cooler and wetter further down. Just don’t plant them too deep or they may never come up!

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  • Plant extra seeds just in case the germination is poor. Just remember to thin them out once the plants are up and growing.
  • Plant them in the shade of taller crops. I usually do my summer planting of radishes in the cooler shade of the pea vines. The peas are fading by the time the radishes are up and growing and need more sunlight.
  • Water daily or even twice a day if it is especially hot and sunny.
  • Use row covers, screening, or shade cloth to provide shade to the bed and protect the tender emerging seedlings from hot sun and drying winds.

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  • If all else fails, start the seeds inside where you have more control over the conditions. Transplant the new seedlings to the garden when they have gotten strong enough and the weather has cooled down a bit.

See which vegetables are best for a fall garden.

See tips for transplanting seedlings.

About This Blog

Get inspired by Robin Sweetser’s backyard gardening tips and tricks. Robin has been a contributor to The Old Farmer’s Almanac and the All-Seasons Garden Guide for many years. She and her partner Tom have a small greenhouse business and also sell plants, cut flowers, and vegetables at their local Farmer’s Market.