Broccoli is a great cool season vegetable. Fresh from the garden, broccoli has a very mild flavor. These broccoli heads are easily 10-12" across.
Spring brings ideal conditions for cool-season vegetables, said a University of Illinois Extension horticulture specialist.
"Cool-season vegetables are able to withstand some frosts so they can extend the growing season in cooler climates," said Maurice Ogutu. "There are cool-season vegetables that are perennial and can live for more than two years in Illinois. There are also annual cool-season vegetables."
Perennial vegetables should be planted at the edge of the garden as they are going to be there for some time. The most commonly grown perennial vegetables in Illinois are asparagus and rhubarb.
"Asparagus should be planted as soon as the ground can be worked in the spring," Ogutu said. "Purchase one-year-old crowns or seedlings from a local nursery or through mail order catalogs. Asparagus plants are male or female and you should select the male plants as they produce larger spears.
"Plant the crowns in an upright position six inches deep in a 12 to18-inch-wide trenches that are spaced 9 to12 inches apart and covered with two inches of soil."
The asparagus spears should be harvested when five to eight inches long from the third year after planting for a shorter period of time. But, in the fourth year and thereafter, spears can be harvested through June.
Ogutu noted that asparagus needs to be fertilized in the spring by applying two to 2 1/2 pounds per square of 100 feet of mixed fertilizers such as 10-10-10 after harvest.
"Rhubarb is a vegetable whose leafstalks are commonly used in fruit pies, tarts, and sauces," he said. "Plant rhubarb roots with the crown buds two inches below the ground in spring as soon as the ground can be worked. The plant requires well-drained soils so you might want to consider planting in a raised bed in areas where drainage is poor."
Plant rhubarb by digging a hole that can accommodate the root as well as crown buds. The plants need to be spaced 36 to 48 inches apart within the row. Because it is a shallow-rooted plant, cultivation should be shallow. Harvest rhubarb during the second year of planting for about a week and for eight to 10 weeks in the third year and thereafter.
"Harvest rhubarb by pulling leafstalks from the plants and trimming off the leaf blades," he noted. "Rhubarb flower stalks tend to develop in spring or summer. They should be cut off and discarded as they can lower the quality of the petioles."
Other cool-season vegetables are grown for only one season, and the majority of them are leafy or root crops.
The root crops include radishes, beets, carrots, turnips, rutabagas, and parsnips.
"These require well-drained soil with plenty of organic matter," said Ogutu. "Radishes are the root crop that matures early. Parsnip matures just before the ground freezes.
"Prepare a fine seedbed and plant seeds of the root crops as soon as the ground can be worked in the spring. They should be thinned to the right spacing when plants are two to three inches tall."
Harvest by pulling the plants from the ground and trimming off the tops when necessary.
The leafy, cool-season vegetables include broccoli, collards, cauliflower, kohlrabi, cabbage, spinach, mustard greens, Swiss chard, lettuce, and Brussels sprouts, which belongs to the Cole crop or cabbage family.
"Plant all of these early in the spring as soon as the ground can be worked or in August for a fall garden," said Ogutu. "You can plant by direct seeding or by using transplants. Transplants are preferred with many leafy vegetables as they establish faster and mature early. Harvest these crops at the right time based on the edible part of the plant by following instructions on the seed packet."
Other annual cool-season vegetables grown in Illinois include onions, potatoes, and peas. Onions can be grown from seeds, sets, and transplants. Onions grown from seeds tend to take longer to mature.
"When growing onions from sets, select smaller sets that are the size of a dime for bulb onion production," said Ogutu. "Larger sets can be used for green onion production as they tend to form two bulbs, and thick necks that are not desirable in bulb onions. Harvest bulb onions when the stems fall over."
Potato stems are called tubers. Plant certified potato tuber pieces with at least one "eye" on them immediately after cutting as soon as the ground can be worked in the spring. Planting time also depends on whether it is a mid-season or late-season variety.
"The mid- or late-season varieties can be planted by the beginning of July for fall harvesting," said Ogutu. "You can also buy already-cut potato pieces from garden centers or seed suppliers. Plant potato seed pieces in furrows two to three inches deep, 10 to 12 inches apart, in rows that are spaced 24 to 36 inches apart.
"After shoot emergence from the ground, hill soil around the plant as the plant grows. Provide plants with uniform moisture, and harvest potatoes after the vines have died by digging with a spading fork. After harvesting, store in a dark room with high humidity and a temperature of 38 to 40 degrees F."
Peas can be planted as early as possible when the soil can be worked in the spring and in July or August for the fall garden.
"Plant pea seeds about 1inch deep and 1 inch apart in rows that are eight to 10 inches apart," he said. "Small seedlings can be injured when they come into contact with fertilizers. Some varieties need to be trained on trellises while others do not need training. Provide plants with adequate moisture.
"Harvesting time for peas depends upon type. Garden peas are harvested when the pods are fully expanded but still immature and should be picked immediately before cooking. The edible podded peas are picked five to seven days after flowering and the pods tend to have higher sugar content. They can be placed in plastic bags and stored for two weeks in a refrigerator, which is longer than the storage period for peas already removed from pods."
More information on cool-season vegetables, he added, can be found at: http://www.urbanext.uiuc.edu/veggies/index.html.