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