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

Tough Weather Week for Vegetables

This past week was a tough one in Front Range vegetable gardens from several standpoints. For establishing tomatoes, we had five nights of overnight low temperatures in the forties F with a sixth night predicted tonight. The official early morning low on June 1 was 42 degrees F. [Postscript - the sixth night June 2 low was 39 degrees F - brrrr!]. This long stretch of lows under fifty degrees F will stop growth of tomatoes and take more days for growth to resume once lows are again in the fifties the first week of June. Can you say lost days for your tomato growing season?

Tomato transplants held for June planting 

Of course if you have your plants in nighttime heating water walls this has not been a problem. The other option some people have taken is keeping their plants "on the shuttle", bringing potted transplants in at night and setting them outdoors by day. June 2 will be the day to plant for those who have waited as this will be the first nighttime low over fifty F. If you timed growing transplants not to become too large, you will have success with holding as plants have stayed in active growth mode.

Remember tomato transplants can be planted deeper than they were growing in the pots and will root out along the stem. This only works if you have a deep depth of decent soil or use the horizontal-planting-with-upturned-top method in shallower topsoil.

Chances are very high the last spring freeze will have passed at this point. Remember the average last freeze in Denver is May 5 with the latest freeze occurring June 8, 2007. For kicks I'll throw in that the latest date of the last measurable snow in Denver was June 12, 1947 (data from NOAA National Weather Service Denver/Boulder office).

The second reason this was a tough week for vegetables was several days of winds in the twenty to thirty mph range. This dried foliage of newly established transplants or germinated seedlings. Careful watering and setting up wind protection structures were in order.

Photo credit: Tomato transplants held for June planting - Carl Wilson

Transplant timing decision & Hardening off

Hardening off tomato transplants near house wall

Betting on when the last spring frost date has past isn't your only gamble in transplanting warm weather tomatoes, peppers, eggplants, etc. These plants are also sensitive to cool night temperatures above freezing but lower than 50 degrees F.

This week with temperatures in the eighties and nights in the fifties F has made it appear that summer has arrived and we're in the clear for transplanting. A closer look at next week's forecast reveals several days of night temperatures expected to be in the mid forties F. Such fluctuations are typical in high elevation, semiarid climates like ours.

Cool nights slow growth  for warm weather plants and recovery takes days. This knocks time off your growing season and raises the question about waiting until night temperatures are consistently warm. I choose to keep plants actively growing by keeping plants indoors and plan to transplant later in the month. Of course if you transplant plants into water walls to raise night temperatures transplanting is a viable option.

Whenever you decide to transplant, do harden off plants by moving them outdoors on warm days and indoors at night. Place where protected and shaded to minimize winds and UV exposure until plants can toughen and develop self-protection against chlorophyll breakdown from UV rays. A location near a house wall under eaves or on a covered porch is fine. A week of hardening off to acclimate transplants to the outdoor environment is usually sufficient.

Photo credit: Hardening off tomato transplants - Carl Wilson

Comparitive Phenology for this Season

Freeze damage on apple. Late flowering
or leafing fruit trees may avoid freeze
damage in most years.

Phenology is the study of periodic life cycle events in plants and animals including such things as date of emergence of plant leaves and flowers. It is influenced by seasonal variations in climate (chief among these temperature) as well as location factors such as elevation.  This year there has been much discussion among gardeners about how "late" the Front Range season is in terms of plant emergence. Is it really?

Phenology is applied to crops in terms of dates that flowers will bloom or crops reach maturity. This is often expressed as a minimum number of "growing degree days", a measure of how much warm weather you have at your site. It's obvious that in springtime this applies more to fruit trees flowering or leafing out than vegetables but it could apply later to vegetables in terms of length of growing season.

So what's the story this year? Are we having an unusually late flowering and leafing out of fruit trees (as well as shade trees and shrubs)?

Looking at Denver weather data and running the formulas, degree days accumulated by the first week of May show we are 1 day ahead of the 30 year normal. Compared to 2012, we are 32 days behind and compared to 2011 we are 20 days behind.

What this means is that we have gotten spoiled. We have been used to plants leafing and flowering 3 to 4 weeks ahead of average so that in an average year like 2013 we think the growing season is off to a late start. Enjoy a normal growing season for once.

Photo credit: Freeze damage on apple - Carl Wilson

New Classes Added!

Thanks to everyone I saw in my April Veggie Keys and Berries and Grapes classes at the Denver Botanic Gardens. The response was so great we couldn't accommodate everyone and DBG has asked me to repeat both classes in May.

As you can see in the class listing at the right, the added "Keys to Home Vegetable Gardening" class will be on May 18th and the "Berries and Grapes for the Front Range" class on May 21st. I hope many of you who couldn't get into the April classes have a chance to join me in May - simply click on the DBG link to register.

I am also teaching a new "Follow-On Vegetable Gardening" class July 10 to help you turn vacant, mid-summer garden spaces productive for fall and winter. Another new "Water-Wise Landscape Planning" class for these drought times has also been added for July 20. I look forward to meeting you in one of my classes.

Hope your transplants are growing well and were snug indoors this week during the 19 degree F record low we experienced in Denver overnight on May 1st. At those temperatures, it even strains the capabilities of water walls to protect early-planted, warm season vegetables. Water walls are generally fine down to the low twenties F. Will the weather soon turn warm? Chances are with us as we get close to our last average spring frost date, May 5. Can you believe the last freeze in 2012 was April 16? Of course in 2010 it was May 13.

Photo credit: Tomato seedlings - 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