Brassica Timing, Temperature & Fertility

Success with Brassicas

Tips on Timing, Temperature, Fertility & Watering

Brassica Trends, Challenges & Rewards

Johnny's has been researching, trialing, and selecting brassicas for the better part of 40 years. As our appreciation grows for the diversity, flavor, and nutritional benefits offered by this family of crops — also known as the crucifers or cole crops — so, too, has market demand for different, better options shot up.

For fresh market growers, providing a steady supply of the new brassica superstars can thus be very rewarding. Brassica production has become increasingly lucrative; nowadays, as a former U.S. president's aversion to broccoli fades into the annals of history, cabbage and kale really do mean "cash."

But growing brassicas successfully can also be challenging. A host of issues can arise if proper attention is not given to timing, temperature, fertility, and watering, as well as pest and disease pressure.

If you're interested in successfully growing brassicas that are not only healthful, but beautiful and flavorful (and, if you grow for a living, profitable), it pays to get familiar with these basics.

Timing & Temperature with Brassicas

Broccoli Harvest from Villageside Farm, Freedom Maine
Broccoli harvest at Villageside Farm, Freedom, Maine

If timing is not everything with brassicas, it's because temperature is just as important. Of all the environmental factors that regulate the development of brassica crops, timing and temperature are among the most critical. More about timing below.

Temperature predominates in the control of the individual plant's developmental transition between the vegetative phase (when stems and leaves, including the tightly wrapped heads of leaves develop) and the reproductive phase (when the heads of flower buds, florets, and flowers develop). Brassicas harvested before this transition include cabbage and kale, for example, and heading brassicas are harvested during or shortly after the process, for example broccoli and cauliflower. When the transition is unbidden or unwanted, a process called bolting, the plant starts to go to seed and the crop becomes unmarketable, as it does with a number of transitional mishaps that can take place, as described below.

Given a grower's limited ability to control temperature in the field, working in tune with nature to get the timing and temperature right is key. Whether the desired stage of your brassica harvest is leafy or reproductive or somewhere in-between, following these tips on timing and temperature will benefit flavor, yield, and quality.

The Case for Brassica Planting Programs

Planting programs are where timing and temperature connect to a grower's best advantage. Planting programs work because different brassica crop varieties perform better in different seasons, or seasonal "slots." Seasonal slots differ in tune with seasonal cycles, as temperatures and daylight are increasing and decreasing. Because of the varietal differences, it is preferable to grow a series of different varieties in sequence, rather than simply repeating the very same variety across the entire breadth of your growing season. This is especially true for broccoli, mini and sprouting broccolis, and cauliflower.

Critical Development Periods in Broccoli & Cauliflower

Planting varieties that are appropriately bred for and adapted to each seasonal window or slot can help prevent some developmental disorders and mishaps.

Crate of Fresh Market Cauliflower
Crate of creamy cauliflower heads
Use our Cauliflower Planting Program to compare and select varieties according to your needs.
  • Riceyness and interior bract formation. When the plants are immature, very warm or hot periods and/or periods of inadequate moisture can result in heads that are malformed: "ricey," pale in color, uneven, and generally unmarketable. If you plan on growing crops in warm weather, choosing varieties appropriate for your area can help you avoid these problems. In our broccoli trials, for example, 'Imperial' has demonstrated the most heat tolerance. In our cauliflower trials, the loose-curd 'Song TJS-65' has tolerated periods of heat better than standard white-stemmed varieties. For more information on cauliflower production in temperate/frost-free zones, see this guide from the University of California.
  • Buttoning up. This term refers to the premature formation of small, unmarketable heads. Buttoning up can be caused by several nights of exposure to temperatures below 50°F/10°C after transplanting. Broccoli and cauliflower are more sensitive to cold than many of their cabbage family relatives. Avoid buttoning by transplanting at the proper time in your area. Adequate water and row covers can also help the developing plants withstand cold spells.
  • Challenges of fluctuating spring temps. There is a reason why so many brassicas are grown in the mild, even-temperature areas of California: most of the varieties available today are bred for these conditions and will not tolerate high levels of heat or widely fluctuating temps. Heat and radical changes in temperature can cause broccoli and cauliflower plants to produce heads that are of low quality and unmarketable. Cabbage, in comparison, is not affected nearly as much by the vagaries of the weather, and will still form good heads except when the heat is extreme.
Purple Sprouting Broccoli
Purple sprouting broccoli
'Santee's' purple florets and green stems are vibrant, sweet, and tender. This variety requires less cold exposure than its forebears among the winter-sprouting broccolis.
  • Getting the timing right for a mature late crop. Whether you're aiming for an early, midseason, or late harvest date, timing is critical. When targeting a late date, planting a bit earlier than ideal is usually better than planting a little too late. When planted a bit on the early side, the crop may be exposed to more later-summer heat than desired, but because this exposure will occur before head formation (for cabbage, broccoli, and cauliflower), the effect of the heat will not usually cause a problem. Planting too late, however, can result in no crop whatsoever, especially if there is an early fall.
  • Vernalization of Sprouting Broccoli. Head formation in what many refer to as the sprouting broccolis, is initiated by exposure to temperatures at, near, or below 50°F/10°C over a period of several weeks. What that means is that these varieties will not form the desired heads until they've been exposed to these cool temperatures, or vernalized.

    'Santee' is one such variety, but with the advantage of having a "low vernalization requirement." While Santee does still need several good weeks of cool weather in the fall, it requires less cold exposure to produce florets than other, more traditional winter-sprouting broccoli varieties. This makes Santee well-adapted for late-fall/early-winter harvest, and it may also be possible — depending upon your latitude and extent of cold weather — for you to harvest Santee into the late winter.

Frost & Flavor

Frost-kissed Kale
Frost-kissed kale


Brussels Sprouts at Farmer's Market
Fresh Brussels sprouts (foreground) at market
The green beauties in the background are 'Vitaverde' and 'Veronica' cauliflowers.

When grown in warmer climes where they are not exposed to cold, many brassica crops are simply not as sweet-tasting as those grown in cold weather. Particularly with kale and Brussels sprouts, exposure to cold is what will set your crop above the ho-hum flavor of those that have not had cold exposure.

How cold is cold, when we say "cold tolerant"?

Cold tolerance and hardiness are a relative terms rather than having precise definitions. A sudden cold snap that occurs in early fall, or before a crop has been exposed to much cold, might severely damage it. The same temperatures in late fall, after much more gradual exposure to cold, will cause much less damage. Furthermore, cold damage is cumulative. Five cold snaps can cause far more damage than one.

Different crops, and within them different varieties, have varying tolerances to cold. Of all the brassicas, the crops with the best cold tolerance include kale, collards, Kalettes, and Brussels sprouts, followed by cabbage, broccoli, and cauliflower.

Brussels Sprouts Planting / Harvest Program

To expand your harvest window, plant several varieties of Brussels sprouts with different maturity dates at the same time, and harvest in succession. The less cold-tolerant varieties are faster growing, and the more cold-hardy varieties are slower-growing. From a spring planting, each variety will mature in its appropriate slot.

Growers in regions with a long growing season can typically make more than one successive planting of 'Diablo,' our variety with the longest "days-to-maturity."

Are Summer Brassicas Worth Your While?

Caraflex Summer Mini Cabbage
'Caraflex': The pointy mini cabbage
The outer leaves of this cabbage protect its tender, crunchy-sweet inner core from summer sun and pests.

So, if brassicas are so temperature-sensitive, is it even worthwhile to plant "summer brassicas"?

Just as with cold temperatures, some brassicas will tolerate a measure of heat — "measure" being the operative word. To decide whether to plant brassicas for summer production and if so, which varieties to plant you'll need to make some considerations: Just how hot does it gets in your area? Do temperatures dip back down at night or do they stay high overnight, and if so, for how many days in a row? What is the approximate, or average timespan over which temperatures exceed the optimal highs for the varieties you want to plant? What lifecycle stage will your crop have reached when temperatures start to climb high? Will the florets have begun to develop yet? And so on.

No one can predict with total accuracy the pattern these variables will form in an upcoming season, but following a planting program and keeping a record of your outcome can help ensure a steadier supply across the growing seasons.

Precisely because there are so many variables we cannot control, we've developed our brassica succession planting programs to work with nature, and based them on data sets we obtain from multiyear, multilocation trials. Our goal is to provide adaptable options that allow you to have a broader, higher-yielding brassica harvest window.

"Heat-tolerant" varieties will do reasonably well in most places, unless heat and humidity are excessive. In addition to the varieties specifically called out in our planting programs, there are some other brassica types and varieties known for good summer performance.

Our sweetest broccoli x gailon variety.
  • Cabbage. Like most brassicas, cabbage prefers cooler temperatures, but Johnny's does offer a number of cabbage varieties suitable for summer growing, so a good cabbage crop can be produced under summer conditions. 'Caraflex', the tender, pointy-tipped type very well known in Germany and Holland, is one of the better summer performers.
  • Collard Greens. This is a crop that is not bothered by heat unless it is excessive, such as in the Southeast and Deep South. In the Deep South collards are typically transplanted in August through late September for fall, winter, and early spring crops. It is usually too hot to grow in midsummer. As noted above, collards also have good cold tolerance, and their flavor can be improved by cooler temperatures.
  • Mini Broccolis. The "summer-sprouting" mini broccolis are generally chosen based on the varietal characteristics we note in our descriptions. For broccoli x gailon, 'Happy Rich' offers best flavor, for example, and 'Atlantis' produces bigger heads and higher yield potential. Of the broccoli raabs, 'Spring Raab' is suitable for early crops: in the different parts of the growing season they perform similarly here in Central Maine, and are generally not bothered by heat unless it is excessive, such as in the Southeast and Deep South (where one just needs to harvest faster than in cooler weather)!

Taking the Mystery out of Brassica Fertility & Watering

Kalettes in the Snow
Kalettes in the snow
The striking result of a non-GMO cross between kale and Brussels sprouts, Kalettes develop in a whorl of tightly-spaced, brilliantly-colored rosettes on stalks of tall, upright plants.

The bicolor rosettes of 'Autumn Star' require a minimum of 110 days to mature, from time of transplanting.

Along with favorable timing and temperature, additional considerations with brassicas include fertility and watering. Pest and disease control are likewise essential to successful cole crop production, and we address the most common brassica pests and diseases separately. Here are the fundamentals of brassica fertility and watering.

  • Steady is key. Plant brassica crops in soils that have adequate levels of nitrogen, where they can be irrigated. Brassicas are heavy feeders and require adequate water to perform well.

    If your soil has adequate fertility, no fertilization will be required during the growing season.

    If your soil is lacking in fertility, then side-dressing with compost or a slow-release organic fertilizer is recommended.

    Taking care not to damage the roots of the plants, hoe in the compost or fertilizer; otherwise, some of the nitrogen will be lost.
  • When to stop. To encourage head/sprout production, or to minimize growth going into dormancy, stop fertilizing in mid summer, so that the plants grow more slowly and "toughen up" for fall.

    Late-summer fertilization will result in plants growing too quickly into fall. These plants will be too lush and tender, and more susceptible to frost and cold damage. This is particularly important for crops such as Brussels sprouts, Flower Sprouts/ Kalettes, cabbage, and sprouting broccoli that will be harvested deep into fall, and even winter or spring.

Brassicas in Abundance

Growers all over the world over many generations have been applying their ingenuity to mitigate the effects of temperature extremes on their brassica crops. For common practices used to offset high temperatures, see Getting Your Cold-Loving Crops Through the Heat of Summer. To learn more about methods employed through the colder months, see Johnny's Winter Growing Guide.

There are also myriad ways to mitigate pest and disease pressure on brassica yield and quality. To learn more, read our review and additional resources on Common Brassica Pests & Diseases.

As more people come to value all the brassicas have to offer, demand for a steady supply of these crops will continue to increase. Each year over many years, the goal of our research, trialing, and selection programs is to offer the most flavorful, best-performing varieties for fresh market farmers and home gardeners. Whether you're new to this vegetable family or a seasoned grower, we hope our selection serves your needs throughout the year. With good planning, quality seed, and sound cultural practices, your brassica program can be very rewarding.