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.

Grow your own again looks big

Last year the nation’s largest seed company, W. Atlee Burpee & Co., reported sales of vegetable seeds up 37 percent. Half of that increase was in sales attributed to new customers (Washington Post, Sep 4, 2008).

This month the big Maine vegetable seed company, Johnny’s Selected Seeds, reports the strongest growth year in their history, 30 to 50 percent. They say their discussions with other seed companies reveal they are up just as much (New England Cable News, February 16, 2009).

The rise in seed sales and it is assumed planting of home and small scale vegetable gardens is variously attributed to desires to:

economize in bad economic times

cope with the high cost of food

produce something to eat from a home that is difficult to sell

deal with food safety worries

seek control in the face of a prolonged recession and job layoffs

engage in horticultural therapy during worrisome times

get back to basics

plant community roots gardening with others as people travel less

use the increased time spent at home in food gardening

reduce their carbon footprint by growing food locally

What can be recommended to these many new gardeners? Front Range Colorado is certainly not their mother’s or grandmother’s gardening circumstances on the East Coast or in the Midwest. In those humid climates growing conditions are more predictable.

Our Front Range area features a short spring for growing cool season vegetables, hot summers with cool nights problematic for heat loving vegetables, and a short growing season. Actual frosts may be a month earlier or later than the average last spring frost and first fall frost. Intense sunlight and dry air both tax plants. Then there are the predominant heavy clay soils discussed in the last post.

Don’t assume that advice given for growing in other climate areas is going to work here. Know the Front Range soil and climatic challenges you are up against. Prepare the soil with amendments, observe narrow planting times and use climate modification measures for a big payoff. Future posts will discuss many of these techniques for success.

[Seed packet photo credit Carl Wilson]

Improving urban garden soils

In nature soils have a well defined top soil rich in microorganisms, plant roots and organic matter. Urban (landscape) soils differ significantly from these native soils. The top soil and other layers are scrambled, organic content is low, and air and water movement is reduced. How did this happen?

During urban construction, thousands of years of soil development is destroyed in moments. The top soil is scraped off, the soil is severely compacted by heavy equipment and structure destroyed. Structure refers to how the various particles of sand, silt and clay fit together, creating pore spaces of various sizes.

Compacted, unamended urban soils typically have a massive structure with no defined top soil, little organic matter, and few large pore spaces. Large pores are where oxygen enters soils for plant root growth. These soils also have few small pores for storing water for roots.

What can gardeners do with their urban soils?
1. Avoid compaction by not walking or running heavy equipment over your garden soil.

2. Improve aeration and drainage through timely cultivation but do not overwork the soil.

3. Most importantly, feed the microorganisms important for plant root growth and function. Their food is organic matter that you can supply by adding compost and other organic soil amendments. Regular additions can boost the typical 1 or 2 percent organic matter Colorado urban soil to the desired 5 percent over time.

Over-amending is a common problem. Some gardeners try to fix their soil by adding large quantities of amendment in a single season. This can create one or more of the following problems:

• High salts
• High nitrogen
• Temporary low nitrogen as microorganisms take it from the soil to break down the organic matter.
• Holding too much water
• High ammonia (burns roots and leaves)

Add a little organic amendment every year until a soil test shows the soil has reached 5 percent. At that point cut back or don’t add any more for a few years. The recommended annual rate is a 1.5 inch depth of plant-based compost (low salt) mixed into soil 4 inches deep, or a 3 inch depth of plant-based compost cultivated in 8 inches deep. That's 4 and 8 cubic yards of compost per 1000 square feet of garden area respectively. Adding more can cause problems such as:

• High salts
• Excessive nitrogen, phosphorus or potassium
• Ground water contamination
• Iron or other micronutrient imbalances

With annual vegetable gardens, take advantage of the opportunity to add a little organic amendment every year. A good rule of thumb without a soil test is to add the recommended rate for 3 years before cutting back by one third in the fourth and following years. A soil test every 5 years or so will tell you how you are doing.

With perennial fruit plantings there is one, up-front opportunity to amend the soil. Don’t overamend thinking that it is your only chance. Overamending will likely create the undesirable problems mentioned above.

Nurture your garden soil and it will nurture the food plants that feed you.

[Soil photo credit, Carl Wilson]

Great Grapes

If you’re thinking about planting table grapes in your Front Range home garden, good thought. They require little care, don’t require much space because they grow vertically, and can be very productive. Everybody in this area thinks of the very reliable Concord variety, but there are alternatives. Other seeded types include Niagra (White Concord), Steuben and Golden Muscat.

I personally prefer the seedless types and really enjoy the harvest from my Himrod vine (pictured above). It’s a white grape. Other seedless types shown to perform well in Colorado State University tests are two whites, Interlaken and Lakemont, and the red grape Suffolk Red. Hopefully the new purple St. Theresa Seedless from Plant Select® will become more widely available and planted over the next few years (photo left used with permission).

Grapes are easy to start from purchased plants, or started from a friend’s rooted cane. Root by lightly scoring and burying the middle of a cane leaving the tip with leaves out of the ground (layering). By midsummer, the middle of the cane has usually rooted. Simply cut the attachment to the main vine and transplant the rooted tip to your garden. Grow on a fence or arbor and enjoy a grape harvest in three to five years.

Although written from a commercial grower perspective, the CSU Extension Colorado Grape Growers Guide publication has useful information for the serious grape grower (click underlined title for pdf file that requires free Adobe Acrobat Reader).

[Himrod grape photo credit, Carl Wilson]