Heat At Last

New grape growth

The average May 2015 temperature in Denver was 4.1 degrees below normal at 53 degrees F. The cool month slowed or delayed growth of some plants and postponed planting of warm season vegetables.

This first week of June turned hot with temperatures in the eighties F. With this warmth grapes finally began to grow in earnest. The late start means flowers will likely escape frost when they come into bloom shortly. This is good news for looking towards fall harvest.

Transplants of squash
and pumpkin vine crops

With increased sunshine soils have warmed over sixty degrees F and vine crop vegetable transplants can be planted.

Warm weather also means it was time to add water to the side channels of Walls O'Water (WOW) to push the tops open. This turns the structure into more of a cylinder than a cone and allows for ventilation while still keeping the plant warm at night.

Wall O'Water with more
water added to open top
into a cylinder

Keeping the WOW on tomatoes or other warm season vegetables is likely wise for another reason. May and June are active storm months for Denver and the Front Range meaning not only rainstorms but also the possibility of hail. WOW offer fairly decent protection from hail.

An understanding of temperature, it's effect on plant growth and what can be done to temper it is a must for gardening success particularly at our high elevation.

Photo credits: Grape growth, Vine crop transplants, Wall O'Water - All Carl Wilson

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

Environmental matters

On a garden tour this weekend I saw tomatoes 4 feet tall with fruit already set. The gardener said she got an early start and used water walls that were lifted off the plants a week ago. Quite a bit different than the June 1st planted plants I have that are 15 inches tall and just starting to flower. Temperature modification for an early start makes a big difference.

Speaking of temperature, both of our plants will be affected by tonight's low that is supposed to be in the forties Fahrenheit. It's always hard to believe that we could have temperatures this cool only a day following a 95 degree day that tied a record. Tomatoes will be stunted for a few days because the nighttime temperature is below the desirable nighttime low of 50 degrees F for tomatoes. This stunting increases the days needed to grow fruit to maturity. This is typical in high altitude areas in the summer and makes tomato growing difficult.

On another subject, the effects of the hail discussed last time continue. The leaves of all types of plants are taking on a faded appearance from the hail nicks scarring the leaves. Plants look similar to when they are in need of water. Check soil carefully before resuming irrigation and if wet, don't water. Plants also may have yellowed lower leaves from roots that shut down due to the excessive rain that came with the hail. Adding more water too early will only compound the hail damage.

It can't be stressed enough to dig down and check soil moisture in the root zone before watering anytime but particularly following heavy rains.

Photo credit: Plant four days after hailstorm - Carl Wilson