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

Rain at last - more needed

The official Denver precipitation total is now up to 0.84 inches for April (still 0.45 below normal for the month though). People closer the foothills received more rain during the upslope weather late in the week. Seedlings are coming up below the poly germination blanket (photo left) even without added sprinkling. See how to use this germination aid in previous post.

The name of the game now is to keep moisture on and through your soil. This is particularly true if you have added compost or turned under a winter cover crop for soil improvement.

Remember that the melding of the organic matter into the soil relies on microorganisms and they aren't active unless there is sufficient moisture. Turned under winter cover crops like rye and Austrian winter pea need a month to break down before planting vegetables IF the soil is moist enough for the microorgaisms to work over the buried plants.

Compost will release nitrogen during the growing season in addition to improving the structure of the soil. Again, soil moisture is needed for this to happen.

Check not only surface moisture, but also subsurface moisture content. After a dry winter, initial rains may have only moistened the top few inches. You may have to add supplemental moisture to wet the soil throughout the soil "profile." How does your soil moisture look down 6 to 10 inches?

Photo credit: Germinated seedlings under blanket - Carl Wilson

Winter garden soil improvement

Vegetable garden soils ideally should contain 5% organic matter. If your soil is lacking, boost it during the winter season by planting a cover crop.

Cover crops are grown to produce green organic matter to turn under the soil. Along the Front Range, plant them from September to no later than mid-October. In addition to boosting organic matter, they prevent wind and water erosion over the winter, build soil structure and suppress weeds.

Legume cover crops can add nitrogen to the soil through the action of nitrogen fixing bacteria. It is good practice to buy rhizobium (beneficial root-associated or “rhizo” bacteria) and apply as specified if the seed is not rhizo coated. Rhizobium bacteria are specific for the legume species and are sold with a specific expiration date.

Broadcast the seed/rhizobium mix at the specified seeding rate and water to germinate. If irrigation systems are shut down, hand watering in the fall will help the crop establish before growth slows for the winter.

Winter ryegrass is often used alone as a cover crop (see photo). Winter rye/Austrian pea or a winter rye/hairy vetch mixture overwinter well in Colorado. Winter rye is a quick germinating and pioneering grass. Hairy vetch is a hardier legume than winter pea for the coldest areas. Seed winter rye and Austrian pea at 4 to 6 ounces per 100 square feet. Plan on 2 to 3 ounces of hairy vetch seed for each 100 square feet of garden area.

Note that these green winter plants are attractive to deer and geese. If they are well established prior to extreme winter temperatures, plants generally recover from winter grazing in spring.

When turned under, the decomposing green material can deprive spring vegetables of oxygen if not done far enough in advance of planting. Plan to mechanically till or hand spade under at least two weeks and better yet a month prior to planting. The legumes may have to be tilled several times to kill them and prevent resprouting.

Many gardeners do not have the land area to plant a cover crop for the growing season without depriving themselves of vegetables for a year. Winter cover crops solve that problem.

Consider planting a cover crop this winter to improve your soil.