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

The Mean Season is Here

The arrival of ninety degree F temperatures this week is tough on Front Range gardeners used to a cool spring but even tougher on plants. Plants can’t get up and go inside for a cool drink when things heat up!

Plants that grew up in a cool environment aren’t tough enough to take hot, dry conditions. Their cells and structure are simply wimpy. Expect to see some heat stress, leaf scorch, blossom drop and bolting in the garden.

To help plants transition to the heat, water with care. Vegetables like most plants need some soil drying between waterings but don’t let soils dry completely. This is especially critical until leaf tissue toughens. Some vegetables (tomatoes) benefit from more soil drying than others (onions and carrots). Bean blossoms (photo right) drop when short on moisture and pods fail to fill; they require the most moisture of any vegetable.

One note on America’s most popular garden vegetable, the tomato. Fluctuations in water supply, either excessively wet or dry will cause blossom end rot. This could have been triggered in June downpours or by July heat. It begins as a light tan, water-soaked spot on the blossom end of the fruit (photo left). These spots turn brown to black and leathery and there is no correcting it once the damage is done. Future blossoms that set fruit when moisture conditions are better won't be affected by damage to previous fruit. To manage this condition, fertilize and water properly. Use mulch to reduce moisture fluctuations.

Want more specific watering tips? Check Colorado State University Extension's Water Conservation in the Vegetable Garden garden note.

Bean blossom photo credit, Carl Wilson
Blossom end rot photo credit, Colorado State University Extension

It’s time to seed fall vegetables

Although it may seem that vegetable gardens have just begun growing well and planting is done, it’s time to seed crops for fall harvest. Sixty day crops planted in July will mature in September as weather is cooling.

Look at seed packets and note the days to harvest. Count back from the first fall frost date, October 11 for the Denver area. With cool season vegetables note that many tolerate the first light frosts of fall very well and an extended harvest period can be expected. Warm season vegetables must be planted to mature well before frost when temperatures remain warm.

Here are some vegetable planting suggestions listed with their typical days to harvest. Cool season vegetables such as peas (65), cabbage (85), collards (55), broccoli (65), kale (60), spinach (40), lettuce (60) and endive (45) are good candidates. Root vegetables such as beets (60), carrots (70) and turnips (50) can be planted this month. Even some fruiting vegetables such as bush beans (60) and cucumbers (55) can still be planted if done by mid-July. The very short season radish (30) can be planted into August with success.

Note that peas planted for fall harvest (photo above) are prone to powdery mildew. Choose powdery mildew resistant varieties. Be sure to provide support such as netting stapled on posts (right) for them to climb to insure good air circulation. Often harvests are reduced as compared to spring plantings. I’m willing to gamble on these Oregon Sugar Pod II’s because I like snowpeas and have a separate area where the air movement is not blocked by tomatoes or other vegetables.

Planting fall crops in “succession” where other spring crops have been harvested is a way to extend harvests past the first frosts and maximize yields from your garden.

Photo credit planted pea seeds and pea netting - Carl Wilson

Got mulch?

With the arrival of temperatures in the eighties and nineties F along the Front Range, it’s time to mulch the vegetable garden. Soil temperatures are now thoroughly warm and mulches will not prevent the soil warming sought in spring.

The use of mulches now has several benefits. It will blunt the force of hard summer rains minimizing crusting and soil compaction. Mulch slows evaporation and can reduce irrigation needed by 25 to 50 percent. Stabilizing soil moisture to avoid wide swings from wet to dry promotes better quality vegetables and encourages the activity of beneficial soil organisms. Mulches applied now tend to keep soil temperatures cooler during summer and suppress weed seed germination.

Weed and seed-free straw (left) and grass clippings make some of the best mulches. Wood mulches are not recommended around vegetable plants, only for paths in the garden. Collect grass clippings from lawns that have not been treated with herbicides (“weed killers”) for at least a month.

Apply a thin layer of grass clippings (right) which will dry in five to seven days (below), then add the next layer. Two to three layers is generally enough to suppress weeds. Do not add thick layers which will mat, smell and limit water and air movement into the soil. Carefully apply around leafy vegetables as grass clippings are difficult to wash from the leaves and heads of greens.

Both straw and grass clippings can be tilled under in the fall and easily break down to enrich the soil. If straw isn’t desired for tilling under due to appearance, it can be raked off and placed in the compost bin.


Straw and grass clipping mulch photo credit: Carl Wilson

Flowers and fruit set of cucumbers and squash

It's time or nearly so when the vine crops will produce their first flowers only to have them drop off. What’s going on?

Cucumbers, squash, gourds and pumpkins produce separate male and female flowers on the same plant. The male flowers on cucumbers (right) appear about 10 days before female flowers and outnumber female flowers 10 to 1. Squash, gourds and pumpkins bear 4 to 10 male flowers for every female flower.

Learn to differentiate male from female flowers, be patient until the female flowers appear, and watch to see if bees are present to move pollen between flowers.

Male flowers are attached by a straight slender stem. The stem of female flowers bears a bulge, the ovary, which grows into the future fruit if pollinated and fertilized. In the squash photo above, the two male flowers (left and center) have just faded for the day and are clearly borne on slender stems. The female (still green) flower on the right will open tomorrow and has the swollen stem bearing the ovary.

Male flowers produce nectar and pollen and female flowers nectar only and in greater quantity than male flowers. Bees gather pollen in early morning and switch to nectar later. Flowers open early in the morning and close by noon or shortly after the same day never to reopen.

Note that the flowers of many other plants produce more nectar and are more attractive to bees than the vine crops. While both honeybees and bumblebees can pollinate vine crops, they can easily get distracted by other flower nectar and fail to pollinate your cucumber or squash flowers. If fruit set is poor you can remove male blossoms, tear the petals off to expose the pollen on the anthers, and hand pollinate.

Cucumber and squash flowers photo credit: Carl Wilson

Strawberry time

June bearing strawberries are coming into their prime (photo right). June bearers fruit over several weeks and ‘Guardian’ and ‘Honeoye’ are good varieties. June bearers are less hardy than ever-bearing types so plant in the warmer cities of the Front Range. You can also mulch crowns in winter after the ground freezes with straw for protection. Like so many other perennial crops, winter watering is useful in dry winters.

Ever-bearers are dependable types for this area and hardier than traditional June bearers. They have two major fruiting cycles with sporadic additional production through the growing season. ‘Fort Laramie’ and ‘Quinalt’ are two commonly available varieties.

Another type to try is the day-neutral berries. They fruit in six-week cycles through the growing season. ‘Tribute’ and ‘Tristar’ are recommended varieties.

Strawberries perform best when planted away from wind in full sun of 8 hours duration. They are sensitive to crown rots so plant in well drained soil with crowns just above the soil. A raised bed with or without hard sides is an excellent growing situation.

I now see many strawberry patches bearing fruit and rewarding gardeners in Denver.

Photo credit – Strawberry fruit on June 10, Carl Wilson