Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu & Emmalea Ernest, Extension Associate-Vegetable Crops; emmalea@udel.edu
Asparagus harvest began last week which is four weeks early. Pea planting is ahead of schedule with heat units accumulating much more rapidly than the thirty year average making it likely that harvest will not extend past June. Peas have emerged rapidly and stands look very good. Average soil temperatures have already been in the mid 60s. Some fresh market sweet corn growers have their first and second plantings in already and sweet corn under plastic has emerged and is growing fast. Plastic mulch laying for summer crops is already underway. As we progress into April, a concern will be the potential for frosts and freezes with advanced crops.
The earliest peas will start flowering at less than 675 heat units and will be at full flower at 775 heat units. Currently, peas planted on February 25 have accumulated 306 heat units. In 2011 during the same period only 181 heat units accumulated. If accumulations continue at an accelerated pace and peas flower early, there is increased risk for frost damage to flowers in early plantings, causing reduced yields, and split sets.
Early planted sweet corn can also be at higher risk this year of frost damage. If the sweet corn growing point is not out of the ground and a freeze occurs, the emerged leaves will be injured but the plants can continue to grow. The growing point of sweet corn is still below or at ground level until the 5-6 leaf stage and is therefore protected against frost injury. If a frost or freeze event does occur with sweet corn, wait 3-4 days to evaluate. If you seed signs of regrowth, the sweet corn can recover. If regrowth is not evident, start splitting the stems of a sample of plants and look at the growing point. If it is firm and white or cream colored it is still alive. If it is soft, water soaked, or gray in color, it has died and you will lose some stand.
Processing sweet corn planting will begin in the next 2 weeks. It is interesting to note that the industry is moving to supersweet varieties for processing. In the past, our recommendations were to avoid early planting of supersweets and wait until soil temperatures were above 65° F, because these varieties tended to have less seed vigor. In 2010 and 2011 we planted supersweet variety trials on April 12. Surprisingly, a number of the supersweet varieties we tested had emergence rates over 90%. This suggests that high quality seed of supersweet varieties with demonstrated cold tolerance can probably be planted earlier, before soil temperatures reach 65°F. Detailed reports on the 2010 and 2011 trials are available online at http://ag.udel.edu/extension/vegprogram/trialresults.htm. This year, early April supersweet plantings may be justified, given that soils have already warmed substantially.
Some rye windbreaks used for watermelon and other vegetable protection have already reached full height and are heading out. Growers may have to kill windbreaks much earlier than normal or realize that seed will likely set which is a problem if rotating into wheat or barley (seed mixing with rye next year). Forage radishes and spring oats did not fully winter kill this year.
As we go into next week, highs are forecast in the 60s and 70s. However the extended forecast for both 6-10 and 8-14 days from NOAA is for “enhanced odds for below average temperature along the Mid-Atlantic”