Saturday, January 30, 2016 By: Ask A Master Gardener

Selecting and Planting Onions in Oklahoma

Time to plant onions is approaching

Bill Sevier:  Ask a Master Gardener

Saturday, January 30, 2016

Q: When is the best time to start onions in my garden? What is the best type? Kim, Tulsa
A: A general rule of thumb is that onions should be planted about 4-6 weeks before the average date of the last spring frost. For our area — because the average last frost is in the first part of April — they are best planted between mid-February and mid-March.
There are many varieties of onions from which to choose. You need to select one rated for our area. OSU fact sheet “Vegetable Varieties for the Home Garden in Oklahoma,” available from the Master Gardener website, lists types that do well in Oklahoma.
Onions are classified several ways. They are grouped by color — yellow, white and purple — and whether they form bulbs. The ones not forming bulbs produce scallions, or green onions. The bulb-forming onions are further divided into the length of days needed to start bulb formation. Long-day varieties are grown in the northern United States; the short-day or intermediate-length types do best in Oklahoma.
Onions are also classified as to taste, ranging from sweet to pungent or hot. This is related not only to the variety but also to the amount of sulfur in the soil. Sweet onions such as Vidalia and Texas 1015 are grown in low-sulfur soils resulting in fewer sulfur chemicals. These chemicals cause onions to be pungent.
Onions are planted as either sets (bulbs) or transplants (sprouts) in rows 1½ feet apart and about 3-4 inches between individual plants. One can plant them closer together and then thin the plants for green onions as they grow. They need full sun and will need ample amounts of water and fertilizer during the growing season. They also prefer loose well-drained soil. If you add an organic fertilizer at the time of planting, this will improve drainage, as well as supply nutrients as the organics decompose.
A generic recommendation for fertilizing onions is to use one cup of a 21 percent nitrogen fertilizer as a side dressing 2-3 weeks after planting and every 3-4 weeks thereafter.
Onions are biennial plants, which means they grow leaves and bulbs one year and then flower and produce seed the following year. If onions grow flowers at the wrong time, it is called “bolting” and impairs their quality. Bolting may be due to stress, or if the onion transplant or set is too large. Try to buy small sets and transplants that are no thicker than a pencil.
Weather is the most common cause of bolting. Onions planted in warm soil begin to grow, and if it turns unexpectedly cold afterward, the plant goes into dormancy until it warms again. This causes the onion to think it is in the next year and “bolts.”
Green onions may be harvested at any point after established. Bulbs onions are best harvested after the tops flop over and turn brown. Most store well when properly cared for. Onions have been grown domestically by man for more than 5000 years and everyone continues to enjoy them.
Garden tips
§  Begin planting blackberries, raspberries, strawberries, grapes, asparagus and other perennial garden crops in February.
§  One can continue, in the month of February, spot-spraying weeds in dormant Bermuda lawn. Use a product containing glyphosate, found in Roundup and others when the temperature is above 50 degrees. Read the label carefully before using.
§  Tomato seeds are best planted in indoor flats around Valentine's Day for mid-April garden transplants. Should you decide to grow your own tomato transplants from seeds consult OSU fact sheet “Growing Tomatoes in the Home Garden” which may be found on the Tulsa Master Gardener web site ( In that same section of the web site, you will find additional tomato fact sheets on growing conditions, pests and diseases which are very helpful.


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