Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Plantings for late summer and fall harvested vegetables will be underway in the next few weeks. Timing these plantings can be a challenge, especially where multiple harvests are needed. Plantings from early July through the beginning of September may be made, with cutoff dates depending on the crop, variety, and season extension methods such as row covers, low tunnels, and high tunnels. These plantings can be divided into 2 groups: 1) warm season vegetables for harvest up to a killing frost and 2) cool season vegetables for extended harvest in the fall.
The three main factors influencing crop growth and performance in the fall are daylength, heat units, and frost or freeze events. A few days difference in planting date in the summer can make a big difference in days to maturity in the fall.
Warm season vegetables for fall harvest include snap beans, squash, and cucumbers. July plantings of sweet corn can also be successful to extend seasons for farm stands. Mid-July plantings of tomatoes and peppers also are made for late harvests, particularly in high tunnels.
Cool season vegetables for fall harvest include cabbage, Brussels sprouts, broccoli, and cauliflower; the cole crop greens, kale, and collards; mustard and turnip greens; turnips for roots; spinach; beets; lettuce; leeks; green onions; and radishes.
To extend harvest in the fall, successive plantings are an option. However, days between plantings will need to be compressed. One day difference in early August planting for a crop like beans can mean a difference of several days in harvest date. Another option to extend harvest in the fall is with planting different maturing varieties at the same time. This is particularly successful with crops such as broccoli and cabbage where maturity differences of more than 30 days can be found between varieties.
Another way to get later harvests is to use row covers or protecting structures (high tunnels). This can allow for more heat accumulation and will aid with protection against frost and freezes. Decisions on what type or combination of covers/protection to use and when to apply the protection will influence fall vegetable maturation and duration of harvest. In general, plantings of cool season crops can be made 30-45 days later in high tunnels than in outside production.
A final factor for summer planting for fall production is on planting cutoff dates. For example, a crop such as cucumber may produce well with an August 2 planting but poorly with an August 8 planting; broccoli has a wider planting window than cauliflower; turnip greens have a wider planting window than kale.
Planting Windows for Fall Harvested Warm Season Vegetables (harvested September through Frost):
Snap Beans: July 10 through August 10
Lima Beans: June 15 through July 20
Cucumbers: July 10 through August 7 (high tunnel transplanted up to September 1)
Peppers: Transplant up to July 10 (high tunnel up to July 30)
Pumpkins and Winter Squash: Direct seed through June 30 (July 10 for short season varieties)
Summer Squash: Direct seed July 15 through August 15 (high tunnel up to September 1)
Sweet Corn: Direct seed July 1 through July 30
Sweet Potatoes: Plant slips up to June 30
Tomatoes: Transplant July 20 through July 5 (high tunnel up to July 30)
Planting Windows for Fall Harvested Cool Season Vegetables (harvest September – December) Note: For transplants, seed 3-6 weeks prior to desired planting date (8 weeks for leeks and onions).
Beets: Direct seed July 1 through August 10
Swiss Chard: Direct seed July 15 through August 20 (high tunnel up to September 30)
Broccoli: Transplants July 15 – August 20
Brussels Sprouts: Transplants June 20-July 10
Cabbage: Transplants July 1 – August 20
Cauliflower: Transplants July 20 through August 10
Kale: Transplants July 15 through August 30, Direct seed July 1 through August 15 (high tunnel up to September 30)
Collards: Direct seed July 15 through August 15
Arugula: Transplants July 15 through August 30, Direct seed July 1 through August 15 (high tunnel up to September 30)
Carrots: June 20 through July 10 (high tunnel through September)
Turnip Greens: August 1 through September 10 (high tunnel up to September 30)
Turnip Roots: August 1 through August 30 (high tunnel up to September 20)
Mustard Greens: August 1 through September 10 (high tunnel up to September 30)
Leeks: Transplant July 20 through August 10
Lettuce (full head stage): Direct seeded August 1 through August 20 Lettuce (full head stage): Transplants August 10 through August 30
Lettuce (baby stage and cut salad mix): Direct seed August 1 through September 15 (high tunnel up to October 15)
Onion (green bunching): Direct seed July 1 through August 30 (high tunnel through September 30)
Parsley: direct seed July 15 through August 15 (high tunnel through September 15)
Radishes (salad): Direct seed August 1 through September 30 (high tunnel through November 30)
Radishes (Daikon): Direct seed August 1 through September 10 (high tunnel up to September 30)
Spinach: Direct seed August 10 through August 30 (high tunnel up to September 30)
Mid-August planted broccoli for fall harvest on white plastic mulch. You can harvest different varieties from the beginning of October to the middle of November.