Growing garlic in Minnesota home gardens
Garlic (Allium sativum L.) is closely related to onions and chives and is used as a medicinal and culinary herb. Garlic forms bulbs, which separate into many cloves, each covered with a white-purplish or pinkish, papery sheath.
Most commercial garlic is grown in the mild climate of northern California. These varieties of garlic will not grow well in Minnesota, and will develop a "hot" flavor. When choosing garlic for your garden, use varieties adapted to cold climates. Note that elephant garlic is a type of leek, not a true garlic.
Garlic varieties are broadly placed into two categories - hardneck and softneck types. Hardneck varieties produce a flowering stalk, called a scape, while softneck varieties do not. However, climate can alter this classification, as a variety that is softneck in one location can form a flowering stalk in a different location! Flowers on scapes usually abort and form "bulbils," or small, aerial cloves. Scapes can be removed just after they start curling, and can be eaten. More mature scapes can be used in flower arrangements. Common hardneck types are Rocambole, Purple Stripe, and Porcelain. Softneck varieties typically produce more cloves and are easy to braid. Softneck types include Artichoke and Silverskin.
Garlic is propagated by planting cloves. Purchase cloves for this purpose from national or local garlic seed producers. Planting cloves from garlic purchased at the grocery store is not recommended. This garlic, primarily softneck varieties, is mainly adapted to mild climates, and generally does not do well under Minnesota conditions.
Optimal shoot and bulb production require a cold treatment, so cloves are generally planted in the fall, usually one or two weeks after the first killing frost. Roots and shoots will emerge from the cloves by the first hard freeze, but shoots will usually not emerge from the soil until the following spring.
Separate individual cloves a day or two before planting. Plant cloves in double rows 6 inches apart within and between rows on beds centered 30 inches apart. Plant cloves pointed side up, with the base of the clove 2-3 inches from the soil surface. Cover beds with 3-4 inches of leaf or straw mulch to prevent fluctuating temperatures during the winter and early spring, and to help control weeds. The mulch can be removed in the spring after the threat of hard freezes is over to help the soil warm up, or it can be left in place to help with weed control and preserve soil moisture.
Soil pH and fertility
Garlic grows best on well-drained, moisture-retentive soils high in organic matter. Well-rotted manure or compost is an ideal soil amendment. The optimum soil pH for garlic is between 6 and 7. Liming is recommended if the pH is less than 5.8. Rates to apply should be based on soil test recommendations (see Understanding Your Soil Test Report). Prior to planting, soils should be well tilled to provide a loose growing bed for bulb growth. Garlic has a moderate to high demand for nitrogen, so urea can be incorporated before planting, and then top dressed as soon as shoots emerge, then 2-3 weeks afterwards. Avoid applying nitrogen after the first week in May, or bulbing may be delayed. Addition of nitrogen in the spring may not be necessary if adequate compost was incorporated in the fall.
Continuous use of high phosphorus fertilizer such as 10-10-10 or 15-30-15, or high rates of manure or manure compost results in phosphorus buildup in the soil. Although phosphate fertilizer applied to soil is bound tightly and resistant to movement in the soil, some runoff may occur. It can then become a major pollution concern in our lakes, rivers and streams. High levels of phosphorus support over-production of algae, which causes significant reduction in water quality (see Preventing Pollution Problems from Lawn and Garden Fertilizers). If your soil tests high in phosphorus, use a low phosphorus (such as 32-3-10, 27-3-3, or 25-3-12) or no phosphorus (such as 30-0-10 or 24-0-15) fertilizer.
Proper watering will enhance good production. Soak the soil thoroughly when watering, to a depth of at least one inch each week during the growing season. There is little or no value in light watering that only wets the soil surface. Sandy soils may require more frequent watering. Stop watering two weeks before harvest to avoid staining bulb wrappers and promoting diseases.
Garlic is a poor competitor with weeds. Unless weeds are controlled early, they can easily overtake young garlic plants, causing significant yield losses. Use of mulch will reduce annual weed competition. A thorough, shallow cultivation before reapplying straw mulch in the spring will keep down annual weed populations. Be sure to use straw free of weed seed as mulch.
Harvesting too early will result in small bulbs, and harvesting too late will result in cloves popping out of bulbs. Depending on variety and climate zone, garlic is normally harvested between late June and late July. One indication to start harvesting is when the lower leaves turn brown and when half or slightly more than half of the upper leaves remain green. Alternatively, you can pull a few bulbs and cut them in half; if the cloves fill the skins, then the bulbs are ready to harvest.
Harvest plants with shoots and bulbs attached. Knock off any large clumps of soil, and then put the plants in a warm, dry, airy place for 3-4 weeks to cure. This will dry the sheaths surrounding the bulbs, as well as the shoots and roots. After curing, the shoots can be cut 1/2 - 1 inch above the bulbs and the roots trimmed close to the bulb base.
Garlic cloves can easily be saved from one crop to the next. It is recommended to keep the biggest one for planting the following year.
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Insects are not a major problem with garlic, although onion maggot is a potential pest. Onion maggots bore into plant stems, causing the plants to turn yellow and wilt.
Garlic is susceptible to several types of rot, Fusarium basal rot being the most common. To avoid these diseases plant only healthy cloves, manage weeds in the garden, and take care not to injure garlic bulbs while working in the garden. Plant garlic in an area where no onions, chives, leeks, shallots or garlic have been planted for the past 4 years.