Dear Tom –
With the recent warm weather we planted peppers in early May but had a scare last week when the temperature dropped to near freezing for one night in a period that had been very warm. I was told our last frost date is May 15. What does that date really mean and when should I set out my peppers?
— Sam in Meadow Fork
Dear Sam –
Our strategy on planting tender summer crops is to keep them protected in the propagation greenhouse until the soil temperature warms enough for them to start actively growing. That approach not only produces healthier plants but often avoids the temptation to plant before the last frost occurs.
When to plant is very site-specific and changes each year. Our farm is just outside Asheville but our temperatures are often ten degrees lower than those in the city or at the airport. We are on the north slope of a mountain and cold air drains across us on clear nights. Last year (2012) was exceptionally warm and sunny in the spring. We planted squash outside in April and were harvesting a few weeks later. This spring has been cool and wet. Even lettuce and brassicas lagged for weeks this year. So the answer on planting date varies not only from place to place but also from year to year.
Here is a systematic process to consider.
- Prepare fields as soil moisture allows in April.
- Warm the soil when you can by using raised beds, plastic mulch, or landscape fabric.
- Monitor soil temperature and resist planting before it reaches the minimum for your crop.
- Keep your transplants in warm conditions (both soil and foliage temperature) until transplanting.
- When the soil is warm enough, check the ten day forecast for daily minimums.
- Double check using the NOAA long term prediction for the next month.
- Plant when both the soil is warm and danger of frost is past.
Deep in the soil, 55 degrees F is typical all year around but soil temperature cycles widely near the surface. We need to determine the temperature that roots will be experiencing once the transplants are planted. Four inches deep is often used but it really depends on how large your transplants are and how deep you intend to plant them. Knotts Vegetable Growers Handbook has a table with minimum (60F for peppers) and optimum (85F for peppers) soil temperatures. For comparison, the minimum soil temperature for lettuce is 35 and the optimum is 75.
These estimates imply that little root growth and plant growth will occur at lower soil temperatures than the minimum. So why risk exposing your peppers to frost when they are more likely to grow in the warmth of your transplant greenhouse? In cool springs we often pot up frost sensitive crops to keep them from becoming root bound and stunted. That step makes the transplants more costly to produce, but hopefully it will produce higher yields when the plants do go into the field.
A USDA site provides soil temperature information http://www.usda.gov/oce/weather/pubs/Weekly/Wwcb/ on a regional basis with some spot temperatures including Asheville. Their report for May 5-11 showed a soil temperature at 4 inches deep of 56 degrees – below the minimum for peppers. A thermometer in your field in the beds that you intend to use for peppers will be much better information. NCSU reports that plastic mulch will warm the soil by 4-5 degrees under black plastic or 8-10 degrees under clear plastic. http://www.ces.ncsu.edu/hil/hil-33.html (Clear plastic is probably not a good idea later in the season because it may heat the soil well above ideal temperatures.)
Knotts Handbook classes peppers as very tender, meaning they will not tolerate frost. Frost will blacken leaves and often kill the plant. I hear the quote that in our area the average last frost date is April 15 with a “guaranteed” last frost date of May 15. That information may come from this table http://nowdata.rcc-acis.org/GSP/pubACIS_results . A few years ago we had a frost on May 27 so that guarantee does not apply on our farm. We keep the row cover handy until June. If you happen to be near the Asheville airport this table of frost dates may be useful http://www.erh.noaa.gov/gsp/climate/newAVLfreezedata.htm Even if your farm is at a higher elevation or farther north, those data may be useful and can be adjusted by a few days. The minimum temperatures at the airport over the period of record are listed at http://www.erh.noaa.gov/gsp/cli/asheville_may.html . It shows 30 degrees on May 18 in 1973.
The 90 day outlook http://www.cpc.ncep.noaa.gov/products/predictions/90day/ provides support to the ten day forecast http://forecast.weather.gov/MapClick.php?CityName=Asheville&state=NC&site=GSP&textField1=35.5741&textField2=-82.5488&e=0 , If we are past our last frost of course depends on your specific location, elevation, and microclimates (such as frost pockets).
So check your soil temperature and consider ignoring the conventional wisdom. In our experience, every year is different. Thanks for your question.
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Tom Elmore is co-owner and operator of Thatchmore Farm in Leicester NC. He has grown certified organic fruits and vegetables for 25 years and serves on the Boards of the NC Greenhouse Vegetable Growers Association and the Organic Growers School.