Learning from the 2013 Gardening Season

The 2013 gardening season was bookended by a very hard freeze on May 1st (19 degrees F in Denver) and a week of rain starting September 9th that soaked gardens and caused record flooding along the Colorado Front Range and plains. The last spring freeze in Denver was May 5th (our average date) and first fall freeze October 4th (October 7th is average date).

Gardeners who planted cool season crops in mid-March achieved a harvest before the May 1st hard freeze although bolting afterwards was an issue. Eighty degree temperatures arrived the last week of May (85 degrees F on May 26) that prompted decline of cool season crops.

For planting warm season crops forty degree lows continued through the end of May and consistent fifty degree F or above nights didn't arrive until June 9th. One 92 degree F day occurred June 3 and then a record 99 degrees F June 10th and 100 degrees F June 11th. Early planting of very warm season crops such as tomato or peppers was best delayed until June or done with water walls if planted earlier.

Vegetable gardeners who haven't considered temperature moderating devices such as water walls until now may want to rethink their approach. It is generally accepted that temperature swings will be more pronounced with worldwide climate change. Take advantage of mulches and season extending devices if you haven't already to achieve satisfactory harvests.

Keep on planting

Follow-on crops to those spring and quick maturing crops already harvested from the garden are the order of late June. Don't let recent above average temperatures in the mid to upper nineties F deter you. If you use hot weather seed germination techniques, summer direct seeding in the garden isn't hard.

Carrot seedlings germinated under
polyester floating row cover fabric.

I recently germinated notoriously slow-to-germinate carrots under germination fabric. This is the same material that many people call floating row cover fabric, simply used for a different purpose. It is readily available in garden centers and will last for many seasons.

Plant seed shallowly as normal, cover bed with cloth, bury edges with soil and/or use bent wire U pins punched through fabric to hold down the middle from winds. Water frequently but in small amounts right through the fabric. Remove fabric upon germination. My carrot seed germinated in 10 days.

If you are interested in more follow-on gardening techniques for late summer and fall growing, join me for my July 10 class at Denver Botanic Gardens (see class list at right).

Photo credit: Carrot seed and Carrot seedlings under row cover fabric - both 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

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