Monday, April 19, 2010

Week of April 19 2010 : Measurements

Instructions on seed packets like "Plant in warm soil" really annoy me. What precisely is "warm"? Or descriptions like "Today is a beautiful day". There is something beautiful about every day, but does an emerging corn seedling agree it is a beautiful day?

So I like numbers and measurements. Temperatures. Growing degree days. Now we are talking specifics.

My trusty soil thermometers measure the soil temperature at 52 degrees. That soil is the coolest due to the cover crop. It retains moisture and shades the soil. At the other extreme are the raised beds; this soil is dry and has no shade. This soil temperature measures 62 degrees. Soil testing convention suggests sampling each morning at 9:00 AM. When the temperature meets or exceeds the minimum planting temperature for three continuous days then the soil is warm enough to plant.

Another fun measurement is growing degree days or GDD. It helps quantify the difference between a day in late April and a day in late July. As the growing season days come to pass, GDDs can be summed together to help predict plant maturity and harvest dates.

For example, sweet corn. Seeds are labeled with relative maturity expressed in days. If the seeds are not also labeled with GDDs, then the number of days can be multiplied by 20 to estimate the GDDs. Applying a little math to the daily high and low temperatures can predict whether the corn will mature ahead or behind schedule. Knowing the average GDDs over the season, you can pick target harvest date to calculate a planting date. You can also use GDDs to properly delay subsequent early season plantings if unusually cool weather delays emergence of prior plantings.

The sum of the average GDDs for the first 18 days of April are ~46. The sum of the actual GDDs for the first 18 days of this April? 197. That is a lot of beautiful days!

Sunday, April 4, 2010

Week of April 5 2010 : Covers

(Here is a picture of our cover crop. Last fall we seeded winter rye, hairy vetch and tillage radishes. The radishes winter killed last December, as expected, but first they established a long tap root. Now the rye and vetch continue to grow. The rye adds organic matter and the vetch fixes nitrogen from the air into the soil. The longer the cover grows, the greater the contribution to healthy soil but also the greater the difficulty working the soil.)

Normally floating row covers are critical to get transplants established in the field. The recent warm, no, HOT, weather seems to eliminate the need for row covers. But we should certainly be properly prepared for cool weather and frost.

Row covers serve two main purposes. They warm the transplants and exclude bugs. During the day, warmer temperatures encourage quicker growth. Warmer nighttime temperatures prevent frost. The covers also prevent pests from reaching the plants.

There are several row cover weights, but we prefer the lighter weight 0.6 ounce per square foot. The trick is to double and perhaps even triple cover the plants early in the season. As the season progresses and air temperatures rise, we remove one layer at a time. For particularly pest-sensitive crops such as cabbage we leave one layer as long as possible. Although this strategy demands more labor, it produces earlier and cleaner harvests.