Quick facts
- Cool-season vegetables include Brassicas (broccoli, cabbage, cauliflower), spinach and lettuce.
- The ideal situation for cool-season vegetables is cool but moderate temperatures for the first month of growth.
- Temperature deviations can cause issues that make these vegetables bitter or cause the crop to fail.
- There are solutions to help get these crops through spring and ensure a good cool-season harvest.
Minnesotans love their cool-season vegetables, including Brassicas (broccoli, cabbage, cauliflower), spinach and lettuce. However, growing these crops in our climate can be a bit challenging. While fall conditions are optimal for most of these cool-season vegetables, Minnesota spring conditions are variable, and often lead to problems like bolting, lack of heads, and bitterness.
Ideal growing conditions for cool-season vegetables
Brassicas, lettuce and spinach all come from the Mediterranean region and surrounding areas, where winters are quite mild by Minnesota standards. They are biennial and winter annual crops, meaning that in their native habitat, they must go through a winter cooling period called vernalization before they can complete their life cycles and produce flowers.
In Minnesota, our spring weather is a lot like winter in the Mediterranean. So it can be tricky to get the right conditions for growing cool-season crops in the spring.
While each specific crop varies slightly, the ideal situation for cool-season vegetables is cool but moderate temperatures (50s and 60s) for the first month of growth. It’s warm enough that the plants are able to grow and put on plenty of leaves, and not quite cold enough to cause vernalization until the plants have accumulated substantial leafy biomass. At this point, once the plant has grown sufficiently large and experienced enough cold, it can shift into its reproductive phase.
Head or root forming vegetables like broccoli or radishes can begin to form their heads and roots at this phase. Ideally, this reproductive phase lasts a while so that the plants can develop large heads and roots before flowering.
Deviations from these weather conditions, though, can cause issues.
Bolting and other problems
Bolting refers to premature flowering. It occurs at any point after a plant has shifted into its reproductive stage. After this shift, high heat and longer days will speed up flowering.
Once a plant flowers, the heads are no longer marketable, and the taste of the foliage often becomes bitter. This is especially true for spinach and lettuce, which both become extremely bitter after flowering.
Buttoning up occurs in Brassica plants that form curd structures, including broccoli and cauliflower. It happens when a plant shifts into its reproductive phase before it’s really ready to support robust heads.
Leafy biomass is what allows a plant to photosynthesize and create energy for itself, and so it’s important for plants to put on plenty of leaves before transitioning to head formation. But when spring weather is quite cool, plants can experience vernalization early, and the result is buttoning up, or the formation of small, often misshapen heads.
Buttoning up can also occur as a result of stress, including nutrient, water, insect or disease stress. This is especially common for plants that are started indoors and held too long before transplanting outdoors.
Sometimes you’ll find large, beautiful green cabbages in your garden that just never form heads, or radishes that seem to be thriving, but when you pull them up the roots are tiny and have not formed a bulb.
This failure to form a head is most commonly a symptom of too much nitrogen.
Other causes include overcrowded plantings (not enough space between plants) and water stress.
Bitterness is most problematic in spinach and lettuce and is caused by a variety of factors.
Primarily, heat and water stress can cause plants to become bitter. As plants age, they become more bitter, and bolting will cause plants to become significantly more bitter.
Solutions to common problems with spring-planted cool-season vegetables
Not all radishes are the same. Some have been intentionally selected for cold tolerance, whereas others have been intentionally selected for heat tolerance or resistance to bolting.
The same is true with all vegetables. Make sure you’re selecting varieties that meet your goals. This is perhaps the easiest and most important way to avoid the common problems listed above.
Most cool-season crops will be marketed according to the best season for planting. Some will explicitly be labeled as spring, fall or heat-tolerant (summer) varieties.
Other seed catalogs will provide a chart showing the best varieties for different times of the year. For later spring plantings, choose a heat-tolerant variety to avoid bolting.
Succession planting is simply the practice of spacing out your planting dates to increase your chances of getting a good crop.
Rather than planting all of your broccoli on April 1, with succession planting you might transplant some of it on March 23, some on April 1, some on April 8, and so on.
Succession planting is great for a few reasons:
- It allows you to spread out your harvest dates so that you can enjoy produce from the garden for an extended period of time.
- It helps to minimize risks during uncertain spring weather. If one succession experiences the right conditions to trigger bolting or buttoning up, the next might not.
Some gardeners like to direct seed their cool-season crops. This is absolutely acceptable and a great option for folks who lack space indoors for starting seeds.
But for spring crops, transplanting can help provide a few weeks of optimal growing temperatures indoors, meaning your plants are likely to mature a bit earlier before the really warm summer weather comes.
There are also some side benefits. For example, transplanted Brassicas are better able to withstand feeding damage from early spring insect pests like flea beetles and cabbage maggots than direct-seeded crops.
Try to minimize transplant shock: avoid planting indoors too early so that the plants do not become stressed prior to planting, and make sure to harden off your transplants.
For eager gardeners who love to get out in the garden as soon as possible, tools that can extend the growing season, like row covers and low tunnels, can help to create a warmer growing environment for early planted vegetables.
Since bolting is partially triggered by the accumulation of cold, these strategies can reduce exposure to cold temperatures for young plants.
Bolting and bitterness can both result from excess heat.
Strategies to reduce heat stress as summer begins:
- Water regularly to keep the soil moist.
- Use organic mulches like straw to keep the soil cool.
The physiology of vegetable crops, edited by H.C. Wien. 1997. (Book)
Bolting in spring vegetables. Benjamin Phillips, Ron Goldy, and Daniel Brainard. 2020.
Reviewed in 2021