Cool Season Vegetables

New Hampshire is famous for its varying weather, especially in spring. Yet this is the time vegetable gardeners are most eager to “get going” and begin planting. There are risks for late frosts and even snow, or long cool periods where many crops would sulk at best. The good news is there are crops which do well in this season.

There are a number of vegetables planted in the cool season that are harvested much later in the year, such as potatoes and onions, but this article focusses on crops that are both planted and harvested in the spring.

There is no shortage of choices for your spring crops.

• The cole crops (or brassicas) are an amazingly large and varied family, whose edible portions span from leaves to flowers to roots. This includes broccoli, cauliflower, kale, cabbage, radishes, turnips, kohlrabi, arugula, Asian greens, and mustard greens (Brussels sprouts, a brassica, are planted in the cool season but take many months to mature).
• Peas (both edible-podded and shelling) are another familiar cool-season crop.
• Lettuce is yet another group that has a huge number of varieties.
• Spinach is also included the cool season assembly.

Memorial Day, always the last Monday in May, is a traditional guide to the last frost date of the season in central New Hampshire. You can adjust this by a week or so depending on where you live, but remember the weather does not always respect the calendar! The temptation is always there to push the season, but doing so can result in a crop failure. Therefore, it is essential to have a guide to when crops can be planted (or transplanted). If you have a warmer microclimate, cold frames, or raised beds which warm up more quickly, you may try to start earlier. Measuring soil temperature is a good technique. Most cool-weather plants will not do well at soil temperatures below 40 degrees F. Oregon State University recommends taking the soil temperature at 9 AM for seven consecutive days, at a depth of two inches, and then averaging the results. A soil thermometer is essential for this.

Temperature is just one factor. Also important is the condition of the soil and if it can be “worked,” typically defined by how wet it is. Though of course the soil must be thawed, it may be too wet for a while. Squeezing the soil in your fist is a good indication, it should crumble and not form a ball.

The following table, which was adapted from Growing Vegetables: When to Plant Your Vegetable Garden, shows the recommended starting and transplanting times for common spring crops.

For 2019, and using broccoli as an example, when is a good time to start seedlings? Memorial Day is May 27. Let’s assume you live in southern New Hampshire, and you think that your last frost date should be a week earlier (May 20). From the chart, you see that two to four weeks prior to that it should be safe to transplant the seedlings. You choose two weeks, which is May 6. You prefer to give the seedlings six weeks to grow inside, so you plant them on March 25.

As you can see, there is a lot of room within the guidelines for personal preference. Your own experience will guide you over the years.

Crops for transplanting should be started in a sterile, “soil-less” medium, provided with adequate light and warmth, and bottom-watered to prevent damping-off. After growing indoors for four to six weeks, transplant outdoors into soil that has been allowed to warm to an appropriate temperature (40°F is the absolute minimum, and higher is better), and use row covers for some protection against a damaging frost or drying winds.  It is a good practice to use at least a 3-year rotation cycle.

Broccoli, cabbage, and cauliflower should be spaced 18 to 24 inches between plants. Kale can be spaced 12 to 18 inches apart.

Asian greens (like bok choi), and kohlrabi, are known to bolt prematurely if they are exposed to cooler temperatures, so they should be set out after last frost. The greens can be spaced 12 inches apart, while kohlrabi requires only 6-inch spacing.

Cool-season crops that are direct-seeded can be sown early, as long as the soil is workable. Lettuce should be planted very shallowly (1/8” deep) as it needs light to germinate. The space between plants depends on the variety—head lettuce should be 12” apart, while loose-leaf can be 6 to 10 inches.

Spinach should be sown ½ inch deep. After emergence the plants should be 6 inches apart. It requires constant moisture so do not let the soil dry out.

Peas should be planted at a depth of 1 inch, and they can be closely spaced. In cooler temperatures they may take more time to germinate. Taller vining varieties will need a trellis.

Turnips are sown at a depth of ½ inch, and spaced 6 to 12 inches apart.