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

Cool, Wet Weather Persists

Freeze damage on tender grape leaves

The limp grape leaves in the photo show one effect of cool, rainy spring weather. Clearing skies on May 10 as a storm system moved out after a week of rain allowed temperatures to plunge below freezing overnight damaging tender new leaf growth. Reserve buds will need to grow to take the place of the lost leaf tissue. Leaves that had already fully expanded and hardened were not damaged.

If you are anxious to plant warm season plants this is a lesson in hardening them off first. Plants purchased at plant sales should be shuttled in and out of the house or coldframe for at least a week, out on warm days and in overnight or on cold days. Nighttime temperatures remain in the forties at mid-month, way too cold for tomatoes and the like. Soil preparation including fertilization and adding compost are good places to direct your efforts now.

Spreading compost on vegetable
garden to mix in 4 to 6 inches

Soil temperatures at 4 inches have crept up to 54 degrees F after plunging to 50 degrees F following a week of cloudy, rainy weather. Keep in mind that the minimum soil temperature for tomatoes is 50 degrees F. Planting is possible under Walls of Water that keep above-ground plant parts warm overnight. Better yet use them in combination with plastic mulch that also utilizes the sun to warm soil for roots to grow. The plastic can be removed in June along with the Walls of Water if you care to not leave in place for the season.

 For some vegetable gardeners warm season crops are the reason to grow a vegetable garden. Cool, rainy spring weather has made this more of a cool season vegetable gardener's year so far.

Delayed planting of warm season vegetables to closer the end of the month is likely a better approach for those not using Walls of Water and plastic mulch. For information on planting warm and cool season vegetables, see the CSU Extension "Vegetable Planting Guide."

Photo credit: Freeze damaged grape leaves, Compost application - both credit Carl Wilson

When to plant vegetables

Warm La Nina effect weather along the Front Range this year has many people anxious to plant. In this post I want to discuss the common wisdom of planting by holidays and the science behind it. As always, individual decisions on planting date are based on risk tolerance. In some years early planting may prove a good bet and in others, an impatient action.

The holiday rule of thumb is to plant peas and lettuce on St. Patrick’s Day and peppers and tomatoes on Memorial Day along the Front Range.

The science behind this is that cool season vegetables are planted when soil temperatures are sufficiently warm for seed germination. These vegetables are able to withstand cold air temperatures. Warm season vegetables require warmer soil temperatures for seed germination and root growth, and warm, stable air temperatures for plant tops that are generally intolerant of freezing air temperatures. Many but not all warm season vegetables are planted as transplants and not direct seeded.

Soil temperatures for vegetable seed germination*

Cool season vegetables –

35 degrees - lettuce and onions

40 degrees – peas, radishes, spinach, cabbage

Warm season vegetables –

50 degrees – tomato, peppers, corn

55 degrees – beans

60 degrees – cucumbers, squash, eggplant

* soil temperatures measured with a soil thermometer at 4 inches at 8:00 a.m.

How does the planting by holidays prescription stack up with the science? It turns out to be a relatively safe guide when used in conjunction with observable weather trends.

Dates for soil warming for years 2005 to 2008*

40 degrees - Mar 28, Mar 28 and Mar 4, Mar 24

50 degrees – May 5, May 6, May 8, Apr 28

60 degrees – Jun 11, May 13, Jun 10, Jun 14

* recorded at the Lory Student Center weather station on the Colorado State University campus in Fort Collins (monitored by the Department of Atmospheric Sciences)

For more information on vegetables and their temperature tolerances, see CSU Extension Garden Notes Planting Guide #720.