The Brassica family, also known as cole crops, includes popular vegetables like broccoli, cabbage, cauliflower, kale, and Brussels sprouts. Achieving a successful harvest relies almost entirely on precise timing because they are cool-season specialists. The window for optimal growth is narrow, requiring plants to mature either before summer heat or before a severe winter freeze sets in.
Temperature Requirements and Hardiness
The vegetative growth phase for most Brassicas occurs best when the average daily temperature remains between 60°F and 70°F. This range supports the development of dense heads and healthy foliage. While plants can germinate at soil temperatures as low as 41°F, they grow much faster when the soil is warmer, around 75°F.
Brassicas can withstand night temperatures below 40°F, especially when mature. However, prolonged exposure to temperatures exceeding 85°F significantly slows growth and can lead to bolting. Bolting is the premature shift to reproductive growth, causing the plant to produce a flower stalk and seeds before a marketable head or dense leaves form. This heat stress results in poor quality, such as yellowing or uneven texture in broccoli and cauliflower curds.
Planting Schedules for Spring and Summer
The spring planting strategy is a race to maturity before the heat of summer triggers bolting. For an early spring harvest, seeds must be started indoors four to six weeks before the last anticipated spring frost date. Starting seeds inside allows the plants to reach a robust transplant size and develop strong root systems before being exposed to unpredictable early spring weather.
Before permanent transplanting, seedlings must undergo hardening off, where they are gradually exposed to outdoor conditions over seven to ten days. This slow transition prepares the plant tissue for wind, direct sunlight, and cooler temperatures, minimizing transplant shock. Transplants are typically ready to be set out in the garden two to three weeks before the last frost date, depending on the crop’s cold tolerance.
Planting slightly before the last frost allows the plants to establish themselves in the cool soil, which is still too cold for many common pests to be active. Successional plantings, where seeds are sown every two weeks, can extend the harvest window. However, all spring crops must be timed to complete their cycle before consistent high temperatures set in.
Calculating the Fall and Winter Planting Window
The fall planting window offers a second opportunity for a successful harvest and often yields sweeter crops because cool temperatures convert starches into sugars. Unlike spring planting, which works forward from the last frost, fall planting requires a backward calculation from the first expected fall frost date (FFD).
To determine the correct planting date, first find the Days to Maturity (DTM) for the specific cultivar listed on the seed packet. Subtract this DTM from the FFD to find the theoretical final planting date. Because daylight hours shorten and solar intensity decreases as fall approaches, plant growth naturally slows down.
You must subtract an additional period, often referred to as the “fall factor,” which is typically 10 to 14 days, from that theoretical final planting date. This extra time compensates for the reduced growth rate in the diminishing light, ensuring the plant reaches maturity before a killing frost arrives. For example, a crop with a 60-day DTM should be planted 70 to 74 days before the FFD. While many Brassicas, particularly kale, can tolerate light frost, the goal is to ensure the bulk of the harvest is ready before a hard freeze damages the foliage.