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 public awareness grows around the tremendous diversity, flavor, and nutritional benefits offered by this family of crops — also known as the crucifers or cole crops — demand for different, better options has shot up considerably.

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

If you're interested in growing brassicas that are not only healthful, but beautiful, flavorful, and — if you grow for a living — profitable, here are some basics on the role of correct timing and temperature, with additional recommendations on watering, fertility, and pest and disease control.

With Brassicas, Timing Is Everything

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

Of the many environmental factors that regulate brassica development, timing and temperature are among the most critical to harvest success.

Temperature strongly governs the transition from the vegetative phase to the reproductive phase — from stems and leaves, including tightly wrapped heads of leaves, to heads of flower buds, florets and flowers. It can also trigger unbidden or unwanted bolting (going to seed) and some other mishaps, as described below.

Given our limited ability to control temperature in the field, working in tune with nature to get the timing 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

Some varieties perform better in different seasons, so it is sometimes better — especially for broccoli and cauliflower — to grow multiple varieties in sequence, than to simply repeat the same varieties throughout the season.

Critical Development Periods in Broccoli & Cauliflower

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 inadequate moisture can result in heads that are malformed, "ricey," pale in color, uneven, and generally unmarketable. If one plans on growing crops in warm weather, choose appropriate varieties for your area. For example, 'Imperial' broccoli, in our experience, has the most heat tolerance. 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.
  • Challenges of fluctuating spring temps. There is a reason so many brassicas are grown in the mild, even-temperature areas of California. They don't like 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 unless 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. Again, timing is critical. Planting a bit earlier than ideal is better than 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 since this exposure occurs before head formation (for cabbage, broccoli, and cauliflower), the effect is usually not a problem. Planting too late, however, can result in no crop whatsoever, especially if there is an early fall.
  • Vernalization of Purple Sprouting Broccoli. Head formation in what many refer to as the winter-sprouting broccolis, is initiated by exposure to temperatures at, near, or below 50°F/10°C over several weeks. Therefore, these varieties will not make heads until they've been exposed to these cool temperatures, or vernalized.

    'Santee' is one such variety. It requires less cold exposure to produce florets than other, more traditional winter-sprouting broccoli varieties, but does still need several good weeks of cool weather in the fall. This makes Santee well-adapted for late-fall/early-winter harvest, but also — depending upon your latitude and extent of cold weather — it may be possible 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 as those grown in cold weather. Particularly with kale and Brussels sprouts, exposure to cold is key to good flavor — and for setting yourself off from the grocery store aisles. Kale freshly harvested during a spell of warmer weather can be bagged and placed in a freezer for a day or two to "sweeten it up."

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

Cold hardiness is relative. 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, 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 the brassicas, the crops with best cold tolerance include kale, collards, Kalettes / Flower Sprouts, 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 plantings of 'Diablo,' our variety with the longest "days-to-maturity."

Summer Brassicas

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, are "summer brassicas" even worth it?

Some brassicas will tolerate a measure of heat, with measure being the operative word. Considerations include just how hot it gets in your area, whether temperatures dip back down at night, the timespan over which temperatures exceed optimal highs for the varieties you have planted, the lifecycle stage your crop has reached when temperatures are high, and so on.

No one cannot predict with total accuracy the pattern these variables will form in an upcoming season, but following a planting program can help ensure a steadier supply across the growing season.

We've developed our brassica succession planting programs to work with nature, allowing for 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. 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 also 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 throughout time have applied their ingenuity to mitigate extremes in temperatures. For a review of these pratices, see Getting Your Cold-Loving Crops Through the Heat of Summer. To learn more about season extension methods for the colder months, see Johnny's Winter Growing Guide.

There are also many ways to minimize pest and disease pressure on brassica yield and quality. To learn more, see our list, with additional resources, for Common Brassica Pests & Diseases.

As more people come to appreciate the flavor, diversity, beauty, and nutritional benefits of brassicas, demand for a steady supply will continue to increase. Each year over many years, Johnny's research, trialing, and selection programs have allowed us to continue to offer the best-performing varieties. Whether you're new to this vegetable family or a seasoned grower, we offer a selection to serve your needs throughout the year. With good planning, quality seed, and sound cultural practices, your brassica program can be very rewarding.