Planting Tips For Cool-Season Crops
This info is partly for Brandy who asked about planting calendars on another thread, but it also is info I've wanted to pull together into one thread for a while because we increase our odds of success with cool-season crops if we plant them when the temperatures are right for them.
This is about cool-season crops. Within the category of cool-season crops, there are two basic sub-categories, although at least one more crop could sort of fit into a third category.
There are cold hardy crops, which are those that can tolerate some sub-freezing temperatures at least down to a certain level, and they can be planted before our average last frost dates. These cool-season crops are hardy ones: aspargus, rhubarb, cabbage, broccoli, kohlrabi, onion, peas, spinach, radishes, turnips and rutabagas.
There are semi-hardy crops, which are those that will be damaged by sub-freezing temperatures, but which otherwise will grow well in cool weather and generally are not harmed by a very light frost or brief periods of freezing temperatures. Cool season crops that are semi-hardy are carrots, cauliflowers, Irish potatoes, beets, lettuce and beets.
Swiss chard, technically speaking is semi-tender and needs warmer temperatures, but it is closely related to beets and in my garden I treat it like a semi-hardy crop. However, it has better heat-tolerance than true cool-season crops.
Each crop has specific temperatures it needs in order to perform best. Sometimes in our highly variable climate it can be difficult to get each crop into the ground at the right time, but we increase our harvest potential when we're able to do so. I plant more by soil temperature, air temperature and 10-day forecast than by a calendar and I base planting decisions on knowing what each crop needs as detailed below.
BEETS: You can sow beet seed into the ground 4 to 6 weeks before your average last frost date, but if you sow it too early, it takes so long to germinate that the seed might rot before it sprouts. For best results, sow your beet seeds after your soil temperature at planting depth has reached 45 degrees and is staying there consistently. For the best-quality crop, plant as early as you reasonably can so your crop can mature before daytime high temperatures are regularly exceeding 65 degrees.
If you plant too early and your plants happen to sprout and grow quickly, they can be "endangered" if you have a long cold spell of a couple of weeks of 45 degrees or lower once your plants are 4-6" tall. This can cause them to go dormant and then when temperatures warm up again they can bolt and go to seed. This is a common problem with all cool-season biennial vegetables.
BROCCOLI: Broccoli produces more reliably when you start your plants inside and set them out when they have 3 to 5 leaves or when they are about 3 to 5 weeks old. Your broccoli will give you the best possible harvest when it grows and matures when temperatures are between 45 and 75 degrees, and in our climate, that's a fairly brief period, which is why it is best to start with seedlings. As with beets, a prolonged period of cold temperatures (40 degrees and below for broccoli) once plants are 4-6" tall can induce dormancy, and then often the plants will form only small button heads once their dormancy breaks. Temperatures below the mid-20s can kill your broccoli plants so try to transplant them into the ground after your 20-degree nights have passed. Warm temperatures can cause your plants to bolt, or flower and go to seed, so you need to plant early enough that your plants have time to form harvestable heads before it gets too hot.
Some forms of sprouting broccoli tolerate warmer temperatures and continue forming small sideshoots in pretty warm temperatures.
BRUSSELS SPROUTS: In the warmer parts of our area, this does better as a fall crop, but in northern parts of OK and northward it sometimes performs well in the spring. Brussels sprouts have very specific temperature needs and will give the best crop if it can mature when temperatures are between 55 and 65 degrees. Brussels sprouts can tolerate cooler temperatures, but doesn't produce well at much higher temperatures. As with other cool-season biennial crops, prolonged temperatures below 40 degrees can induce bolting, but otherwise the plants themselves are cold hardy to temperatures down to about 20-degrees. It is better to start with transplants as that helps increase your odds of harvesting a good crop before the heat arrives.
CABBAGE: Cabbage is very cold tolerant and can handle temperatures down into the low 20s, but if direct sown from seed, germinates and grows best once soil temperatures are at or above 50 degrees. This is one reason many people start transplants indoors where the seeds will germinate more quickly than those sown directly into the ground. As with other cool-season crops, long-term exposure to temperatures below 40 degrees once the cabbage are a certain size can induce dormancy followed by bolting. If temperatures get too hot too fast, the heads can become puffy and misshapen although I haven't noticed that often, even with cabbage that isn't harvested until June or July.
CARROTS: Carrots have a reputation (somewhat justified) for being hard to grow, but they are not that hard if you are able to meet their needs. Carrots produce best and have the best flavor if they grow and mature when temperatures are between 45 and 85 degrees. Generally you want to plant your seed about 90 days before your daytime high temperatures begin regularly exceeding 85 degrees. Unfortunately, in our climate those mid-80s can arrive pretty early and carrot seed can be slow to sprout in cool soils. Sow your carrot seed after soil temps have warmed to 45 degrees. It is important to keep the soil moist, but not soggy, and one way to do this is to place a board, a piece of cardboard, or a sheet of plastic over the bed where carrot seeds have been sown. Check underneath it daily, mist the soil surface lightly and remove the covering for good once you see the earliest sprouts. Carrots are fairly cold tolerant, but temperatures in the low 20s can kill the plants. Like other cool-season biennial crops, carrots can bolt if subject to long-term temperatures below 45 degrees while already a good sized plant. If you leave your carrots in the ground too long, even though they seem fine, their flavor will be negatively impacted and they may become tough.
CAULIFLOWER: This does best in southern OK as a fall crop, but folks from central OK northward might have more luck with it as a spring crop than we do down here. Cauliflower produces the best-quality harvest when the heads mature before daytime highs begin regularly exceeding 75 degrees, which can be a problem in areas where your temperatures warm up very quickly. Some newer hybrids mature more quickly than older OP varieties and may increase your chance of success. Because of the heat issues, Cauliflower is easiest when grown from transplants and can tolerate very cool temperatures if well-hardened off.
COLLARDS/KALE: These grow best between 45 and 75 degrees but will tolerate a very wide range of temperatures. You can direct-seed collards and kale seeds once soil temperatures reach 40 degrees. For many people they grow better as a fall crop than as a spring crop, although in long cool springs they perform well. Even though they can
"survive" in warm temperaturs, their flavor can become very hot or strong. Often they will overwinter (might not have done so in NE OK last week!) if planted in fall a month or two before your first fall froze. You can plant them in fall, harvest from them during the winter and into spring, but warm spring temps will cause them to bolt.
KOHLRABI: Technically a cool-season temperature but can tolerate pretty warm temperatures. The best quality harvest will come before daytime highs regularly are exceeding 75 degrees. You can direct-sow kohlrabi after soil temps are at 45 degrees or above. Temperatures below 20 degrees can kill kohlrabi plants and, like other cool-season biennals, prolonged exposure to temps below 40 degrees once the plants are a certain size can cause bolting.
LETTUCE: In our climate, lettuce is strictly a cool-season crop although some people grow it indoors year-round. You can direct-seed lettuce about 4 to 6 weeks before your average last frost date although temperatures in the mid- to upper-20s can kill some varieties. A few varieties are somewhat more cold-tolerant. Very few lettuce varieties have any heat tolerance in terms of what we consider "heat" here in this part of the country. Those lettuce varieties described as "heat-tolerant" may be hate-tolerant in Pennsylvania or Michigan or New Hampshire, but onlly because their summer heat is not as extreme as ours. The most heat-tolerant lettuces I've found are those from very hot parts of the world, like Australia or Israel. Your best quality harvest will be that harvested before your daytime high temps are exceeding 80 degrees.
MUSTARD: Mustard greens need to mature at cooler temperatures are the flavor can become more strong than most taste buds can bear. Plant mustard greens about 3 to 4 weeks before the last killing frost, and aim to harvest your crop before daytime highs begin regularly exceeding 70 degrees.
PEAS: This refers to English peas, snow peas and snap peas. All other peas like black-eyed, purplehull pinkeye, cream, zipper and lady peas are warm-season crops generally referred to as "southern peas".
Peas are highest in quality when they mature before daytime highs regularly exceed 75 degrees. You can direct sow them but they can be so slow to germinate in cool soil that they rot before sprouting. Pre-sprouting them indoors in a damp coffee filter or paper towel placed in a ziplock bag can help work your way around that issue. You should plant your peas late enough that they receive as little exposure as possible to temperatures in the low- to mid-20s. While small, pea plants tolerate cold temps and even snow, but once the plants are blooming, freezing temps can freeze back the tips of the plants and can knock the blossoms off the plants. Smooth-seeded varieties are more cold-tolerant than wrinkle-seeded ones.
POTATOES: This refers to Irist potatoes and not to sweet potatoes, which are a warm-season crop. Potatoes grow and produce best when they have nighttime lows between 45 and 55 degrees and daytime highs between 60 and 75 degrees, a period of time that is all too brief in our climate. Plant your seed potatoes in the ground about 4 weeks before your average frost date. Late plantings will produce smaller crops because your plants need to set and size their tubers before your temps are hitting 85 degrees. Heavy mulching can keep the ground somewhat cooler, but don't mulch until foliage is up above the ground.
RADISHES: These are very easy. You can sow radish seed once your soil temp is exceeding 45 degeres and can continue to succession every week or two until about a month before your daytime highs begin exceeding 80 degrees. Radishes harvested once temps are that high often have a strong, unpleasant flavor and pithy texture.
SPINACH: A true cool-season crop, spinach matures best before daytime highs regularly exceed 50 degrees, so often performs better here as a fall crop than a spring one. Spinach seed sprouts best once soil temps are 45 degrees or warmer, which is one reason it is hard to get a spring crop.
TURNIPS/RUTABAGAS: These grow best at temperatures between 40 and 80 degrees. You can sow the seed pretty early in spring or even in late winter but not so early that your plants will be exposed to temperatures below 20 degrees.
ONIONS: These have very specific needs and you must plant precisely at the correct times in order to maximize your harvest. You want to plant as early as possible so the plants can get as large as possible before bulbing is initiated by daylength. This is where onions from transplants have the advantage over those sown from seed in our climate. Generally, the best time to plant is 4-6 weeks before your average last frost date.
Many people experience trouble with onions bolting(flowering and going to seed). As with other cool-season biennial crops, this occurs once plants of a certain size are exposed to temperatures below 45 degrees for about 10 to 14 days. The size? Roughly one-quarter inch, or about when the plant has formed 6 leaves. There is little you can do to avoid this other than planting at the proper time, planting at the proper depth (planting too deeply harms them) and hoping for consistently cool weather. Always choose the right type of plants for your climate, selecting either short day, intermediate or long-day types as recommended for your specific region. Onions bulb when daylength (i.e. number of hours of daily sunlight) reaches a certain level no matter when you planted them. So, earlier planting (as long as it isn't so early they'll freeze or bolt) gives you the best chance of raising nice big onions.
In our climate, you just have to accept that some years the weather is so erratic and bolting will occur.
I'm linking Tom Clothier's seed germination/temperature data base as I often do when we're discussing seed-starting and germination issues. It is full of useful information, but you cannot go to it and just pick the best germination temperature that gives you the highest percentage of seed germinated in the shortest number of days. Why? Because if you do that, you'll be planting much too late for many cool-season crops. Carrots, for example, germinate best at 77 degrees, with 96% of the seeds germinated in 6 days. However, carrots grow and produce best when grown at air temps between 45 and 85 degrees, and if you wait until your soil temp is 77, your air temps likely will be in the upper 70s or 80s already and soon to move into the 90s. So, you have to compare the temperatures each crop needs for good growing conditions to the seed-germination temperatures and choose a temperature that will get your crop growing in time to take advantage of the best growing temperatures.
Hope this info is helpful.
Here is a link that might be useful: Seed Germination Temperature Charts