Weather Changes to Too Hot

In typical high altitude fashion the weather has gone from too cold nights to too hot days for vegetable crops along the Front Range of Colorado. A new record high of 99 degrees F was reached in Denver on June 10 and more temperatures in the nineties are expected to follow the rest of the week.

Lettuce now bolting

Tomatoes will stop growth at temperatures over 95 degrees F. These hot days have arrived just when we have plants established and want them to grow vegetatively to develop a good sized frame to set blossoms. Moderate temperatures between 65 and 85 degrees F are best for tomatoes and the last few weeks illustrate why the Front Range is less than ideal for growing them.

The cool season crops will quickly wane with hot weather. Spinach bolts and goes to seed at high temperatures especially under long days. High temperatures cause lettuce seedstalks to develop and quality to decline quickly as this photo taken a few days ago shows. 

'Indian Red Giant' mustard starting to bolt  -
 photo taken two days before post

Several of the oriental brassicas have a tendency to bolt under the following conditions. Low temperature in the early stage of growth is thought to be the single most important factor. If seedlings log enough heat units before the low temperatures, bolting is prevented. The long days of June are another risk factor. Stresses such as transplant shock, lack of or excess water, and temperature shocks increase bolting risks.

Once bolting begins, salvage leaves immediately before quality declines further. Choose bolt resistant varieties next time or use bolt-prone varieties as cut-and-come-again seedling crops to avoid the bolting issue.

Low humidity and drying winds of 10 to 30 mph this week will make conditions difficult for young vegetable plants and seed germination in progress. Frequent, light waterings and wind protection if available are in order.

Photo credit: Lettuce bolting and 'Indian Red Giant' mustard bolting - both Carl Wilson

Asian Greens

If you haven't investigated the world of Asian greens, they will surprise with their versatility and tastiness. The striking appearance of many are a welcome addition to the garden. Cool season types such as those mentioned here can be planted now.

'Brisk Green' pak choi

Pak choi (a.k.a. pac choi, bok choy, Chinese celery cabbage, Chinese white cabbage) Brassica rapa var chinensis, is generally a cool season vegetable though improved heat tolerance has been bred into some. The green leaf stalk types like 'Shanghai' and particularly the F1 varieties 'Mei Qing' and 'Brisk Green' with more heat and bolt resistance are good candidates for us. They can be sown in place and the young greens cut in 20 days while thinned plants left to mature in 45 or 50 days depending on variety. Pak choi may be eaten raw, stir fried, grilled, steamed or added to soups.

Pak choi 'Brisk Green' bolting mid-June

Seed of pak choi is available from general seed companies such as Johnny's, Nichol's Garden Nursery and Territorial as well as specialty Asian vegetable seed suppliers like Kitazawa Seeds. Sow directly in the garden in April and again in early July for a fall crop. As with other cabbage family members, flea beetles and cabbage caterpillars can be troublesome. Try floating row cover fabric to screen them out.

'Indian Red Giant' mustard

Asian mustards, Brassica juncea, are fast growing cool season crops. A particular beauty is 'Indian Red Giant' mustard, 40 days to maturity or half that time for baby salad greens. This maroon leafed plant forms a welcome addition to lettuce or spinach mesclun mixes and can also be used as wilted greens and in soups and other cooked dishes. The peppery flavor and texture of the leaves are very different than American mustards. Originally thought to come from India, it is popular in many places in Asia. Seed can be obtained from local Broomfield based Botanical Interests.

Photo credit: All story photos credit Carl Wilson