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

Bees to pollinate your Vegetables and Fruits

Digger bee coated with pollen - Whitney Cranshaw

Devoting valuable growing space to flowers to nurture bee pollinators may seem like a waste of time to vegetable and fruit growers. Perhaps it's time to rethink and plant some low-care perennial flowers for bees.

Pollinators have been in decline with increasing urbanization and the mysterious honey bee colony collapse disorder. We need bees to pollinate our food. Vegetable growers need to be concerned about pollinators for their vine crops: cucumbers, melons, squash and pumpkins. Fruit growers are even more dependent on pollinators for brambles, strawberries and tree fruits.

Sweat bee and honey bee - Whitney Cranshaw

Although you may automatically think of honey bees, don't sell bumble bees and solitary bees short when it comes to pollination. There are a whole variety of native solitary bees to consider.

Some of the better low water and low care perennials to consider growing for bees are:

• Sunflower (Helianthus sp. but not pollen-free florist types or fancy doubles)
• Catmint (Nepeta - extend flowering by cutting back after first flowering for a second flush of bloom)
• Beardtongue (Penstemon sp. including Rocky Mountain penstemon, P. strictus)
• Russian sage (Perovskia atriplicifolia)
• Mint (Mentha sp.)
• Purple coneflower (Echinacea purpurea)
• Blue mist spirea (Caryopteris x clandonensis)
• Lavender (Lavandula angustifolia)
• Silverheels horehound (Marrubium rotundifolium)

Also consider succession of bloom over the growing season. Don't know when perennials bloom? Click to find a helpful publication from Utah State University Extension complete with flower bloom time chart that is highly applicable to us in Colorado:  Gardening for Native Bees in Utah and Beyond.

Transplant Seeding

The time is near to start seed if you're planning to grow transplants of tomato family plants. Count backwards from your desired date to plant out your transplants to determine a seeding date. Beginning gardeners often start plants too early and end up with overly large plants of declining quality that have outgrown their pots.

Tomatoes require 5 to 7 weeks to grow a transplant. Peppers and eggplants require slightly longer, perhaps 7 to 8 weeks. Remember to allow a week longer if growing on a windowsill or under low light conditions. In a greenhouse with good light and temperature control, a shorter growing time is needed. If planning to transplant out the last week of May in Denver, the first or second week of April is a good target date for seeding.

Other typical transplants, primarily vine crops, should be seeded later. Squash, cucumber and melons require only 2 or 3 weeks to produce the small-sized transplant needed. The root system on large vines does not transplant well on these crops.

Just a note if you haven't already acquired seed of tomatoes. Tomato Fest out of California is a good source of heirloom tomato seeds that is not well known in Colorado. This is my source for seed of 'Azoychka', a yellow tomato that is well adapted to our area as I've written in previous posts.

'Azoychka' (see photo)  produces 3 inch tomatoes with good acid to sugar balance and nice citrusy flavor notes for those who like yellow tomatoes. It is a 70 day indeterminate type that comes from Russia. This variety rated in the top ten heirlooms sold by Tomato Fest and deserves a try in your Front Range garden.

Photo credit: Planting seed to grow transplants and 'Azoychka' tomato, both Carl Wilson