Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Late plantings of summer vegetables will go in the field in July and early August. Cool season crops will also be planted at this time for fall harvest. The following are some guidelines for these plantings.
With sweet corn, growing degree days become limiting with plantings after August 1. With an 80 day corn that requires 1400 GDD to harvest, a July 15 planting will be ready to harvest before September 10. A July 25 planting will not be ready until after September 20, and an August 1 planting will not be ready until early October. Past August 5, the risk of frost before corn matures is high. Late plantings will need to be closer together than spring plantings to account for reduced heat unit accumulations that occur as days shorten later in the year.
For cucumbers (pickles and slicers), the latest planting date for Delmarva should be August 7. Experience has shown that after that date yields drop considerably. With summer squash, late plantings can be made until August 10. Pumpkins will need to be planted by July 5 to hit the October markets. Cantaloupes and specialty melons do poorly in cool nights and should not be planted after July 5 unless they are being grown in a high tunnel. While watermelons can be planted up to early July, there are limited markets after early September so last plantings are made in June.
Late tomatoes in the field should be planted by July 10 unless going into a high tunnel. High tunnel tomato plantings for fall harvest should be made by July 20 for best yields. Peppers for fall harvest can be planted in the field up to July 25 and in a high tunnel up to August 10.
Snap bean crops can be planted up to August 10. Baby lima beans should not be planted after July 20 and Fordhook types should not be planted after July 5.
Cole crop planting for fall harvest will begin soon using transplants. Broccoli is transplanted from July 15 to August 20, cabbage from July 15 to August 10, and cauliflower from July 15 to August 10.
Kale and collards should be seeded before August 15 for best yields, transplants can go in up to August 30. Turnips and mustard greens can be planted from late July through the first week in September.
Beets are best planted before August 10 for roots, carrots should be planted by July 5.
Green onions should be planted by the end of August for fall harvest. Overwintering green onions can be planted through October using hardy varieties.
Bulbing onions for overwintering should be seeded from September 1 through September 15 using overwintering specific varieties. Bolting will be a problem if planted too early. If using transplants, planting should not take place until October. Leeks are transplanted from August 1 through August 20. Garlic cloves are best planted November 1 through November 20.
Spinach for fall harvest is seeded August 10 through the end of August for fall harvest and from October 1 to October 20 for overwintering.
Lettuce for heads from direct seeding should be planted during August. Transplanted lettuce for heads best planted from August 10 to September 10. Fall adapted varieties are required. Earlier plantings may be subject to bolting and only bolt resistant varieties should be used. For baby lettuce, field plantings can continue through mid-September.
For all late crops, variety selection is very important. For example, late planted cucurbits such as squash need broad virus resistance. For fall harvested crops, switch to shorter maturing varieties as plantings gets later. When planting late, remember that a few days delay in planting can mean several weeks later harvest. A longer maturing variety may not produce if planted too late. On the other hand, planting several maturities of varieties on the same day will often give long extended harvest in the fall for crops such as broccoli.
For crops that are transplanted, transplant production will need to start 4-6 weeks before planting. For many of our late planted crops, we are already too late to produce transplants.
High tunnels, planting on black plastic, and using row covers will often require changes in planting dates. For example, lettuce and other leafy greens in high tunnels can be direct seeded through early November. Overwintering crops in high tunnels should be planted much later than field plantings.