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

Prepare for succession planting

Buy your seeds now to prepare for second plantings following harvest of lettuce, spinach, peas, kale and other spring crops. Bolting (seed stock formation) of cool season spring crops is a common sight following the first hot temperatures. See lettuce bolting photo left and spinach bolting photo right below. These plants should be cleared to make room for other vegetables to maximize garden productivity.

Planting of warm season vegetable transplants of tomatoes and peppers and seeds of squash , corn and beans may replace some of these spring crops now that soil temperatures have thoroughly warmed.

Do remember the mid-summer planting opportunity for fall crops of cool season vegetables. Early to mid-July is the time to seed your fall crop of kale, lettuce, peas and other approximately 60 day cool season vegetables. Even though they will be germinating and starting growth under high summer temperatures, they will be maturing in September when temperatures are cooling. Quality can be good as a result. Note that there is also a fall opportunity to plant leafy vegetables desired for early spring crops. See the Colorado State University Extension Garden Notes #719, Vegetable Garden Hints.

Shop for seeds now to have them on hand before seed supplies run out and seed racks disappear until next year.

Lettuce and spinach bolting photos credit, Carl Wilson