Wednesday, July 15, 2009

The Challenges of Summer Sowing

Mid-summer sowing of vegetable seed can be a challenge due to weed competition and warm soil temperature.

Weeds germinate quickly in summer and it can be difficult to tell the difference between weed seedlings and vegetable seedlings. One tip is to clear the soil surface of weed seeds by germinating and eliminating them. A week or more prior to sowing, water soil to germinate summer annual weeds such as purslane (see photo below left) and crabgrass and foxtail (photo below right).

Next use a sharp scuffle hoe to cut off the weeds from their roots, disturbing the soil as little as possible. Deep penetration of the soil will bring up additional weed seeds to germinate. Once the soil surface is cleared of weed plants, sow small seeded vegetables at the recommended shallow depth. Use germination fabric over them during germination (see March 6 post for how).

Warm soil temperatures can be a concern for germinating some seeds. The maximum soil temperature for germinating lettuce and spinach is 70 degrees F. The optimum temperature for germinating peas is 70 and many other vegetables 80 degrees F. Seeds become dormant at high temperatures.

Tips for avoiding warm temperature dormancy are to water soil before sowing and sow in the late afternoon so germination will begin overnight when soils are cooler. Shading the soil with seed germination fabric also helps.

Another tip is to pre-germinate seeds. The pea seeds (photo left) were soaked overnight indoors at room temperature (70 degrees F) in a moistened paper towel enclosed in a plastic bag. Do not soak longer or delicate seedling parts will be damaged in planting.

Pre-germinating small seedlings is more difficult. Solve this through fluid seeding. Germinate small seeds on a moistened paper towel in a closed plastic container kept indoors at 70 degrees F. Don’t allow seeds to germinate for more than a few days, only until they just break out of their seed coat. Wash germinating seed off the towel into a sieve with a gentle stream of room temperature water.

Gently stir germinated seeds into pre-prepared fluid seeding gel. The gel is made by dissolving 2 to 3 tablespoons of cornstarch in boiling water and cooling to room temperature. Add the gel-seed mixture to a plastic bag, seal, snip off one corner and gently squeeze the gel onto moistened soil. Cover as recommended and use seed germination fabric to shade. Keep fabric in place until green seedlings are readily apparent. Remove fabric and water carefully until seedlings are growing vigorously.

Crabgrass, purslane and soaked pea seed photos credit – Carl Wilson

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