Want to grow great garden onions this summer? Start now
In today's "Growing Together" column, Don Kinzler says growing onions isn't difficult, but following a few specific guidelines produces a heavy yield of large bulbs.
How is an onion like a tightwad’s wallet? He cries when either is opened.
Speaking of onions, I grew up in a German household where potatoes, cabbage and onions were mealtime staples. No hotdish was complete without onions added, and we grew enough in the garden each summer to last through the winter, stored snuggly in the root cellar.
Growing onions isn’t difficult, and following a few specific guidelines produces a heavy yield of large bulbs. The planting season still seems distant, but a good crop of onions can begin now.
Onions are classified as either long-day or short-day types. For the Upper Midwest, select varieties labeled long-day, which is easy to remember because daylength is long during our growing season, as opposed to areas of the country where winters are mild, and onions are grown during the season’s short days.
There are three ways of planting onions in the garden. You can plant seeds directly into the garden soil, but onions require a long period of growth, and direct sowing usually yields small bulbs suitable for fresh green use, but not full-size storage onions.
Dry “onion sets” are a second planting option offered at garden centers in spring, and this is a common choice for many gardeners. Onion sets are small, dry, dormant bulblets that are planted in the garden in April or May and yield decent-sized bulbs. Some varieties have good storage ability.
But if you really want magnificent onions, the third planting method is the key. The largest onion bulbs result from transplanting pre-started plants into the garden. Many garden centers sell onion plants, either in bundles, or planted in trays and packs. You can also start your own onion plants from seed indoors.
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It’s not difficult to grow onion plants for spring garden transplanting, and the following are useful tips:
- Onions grow slowly, so they're best seeded indoors by the end of February or before. The earlier the seeding, the greater the potential for large onions.
- Select long-day types. Cultivars recommended by North Dakota State University include Ailsa Craig, Candy, Copra, Sweet Sandwich and Walla Walla. Of these, Ailsa Craig produces the largest bulbs, but it lacks long winter storage ability.
- One of my personal favorite onion cultivars is Dakota Tears, developed by Prairie Road Organic Seeds in Fullerton, N.D. This North Dakota seed company bred Dakota Tears for large size, great flavor and ability to store all winter.
- Onion seeds can be started in shallow trays, greenhouse flats or repurposed clear plastic containers from grocery store baked goods. Drill or poke drainage holes in the container’s bottom.
- Use seed-starting germination mix, which is milled finer than all-purpose potting mix. Moisten and stir the mix before placing in the seed tray.
- Plant onion seed shallowly, covering with about a quarter inch of potting mix, and water gently.
- Cover the tray with plastic, or the container’s clear lid.
- Warmth will speed sprouting, such as on a germination mat, but onion seed will germinate under cool temperatures also.
- Provide full light immediately upon sprouting, such as a sunny window or under lights which are set to run at least 16 hours per day.
- If seeds were planted thickly, plants should be thinned to a half-inch apart while young.
- Continue to grow, fertilizing seedlings with half-strength water-soluble fertilizer every two to three weeks.
- Transplant onion plants into the garden in late April or early May. They can withstand light frost.
- If soil is hard-packed, amend with 2 inches of peat moss or compost worked into the top 6 inches of soil before planting.
- When transplanting into the garden, trim tops to 4 inches long.
- Plant 3 to 4 inches apart in double or single rows spaced 12 to 16 inches apart.
- Plants that grow healthy and large are the surest route to harvesting large onion bulbs. Each leaf produces a layer within the bulb, so the more leaves, the more layers.
- Good nutrition increases size. Soil tests can indicate present fertility. As a general rule, apply a half cup of 10-10-10 granular fertilizer per 10 feet of onion row.
- Onions are shallow-rooted and struggle when the soil surface is dry. Onions require 1 inch of moisture per week, either from rainfall or sprinkling. Mulching between rows can conserve moisture. Keep mulch several inches away from the onion plants.
Don Kinzler, a lifelong gardener, is the horticulturist with North Dakota State University Extension for Cass County. Readers can reach him at firstname.lastname@example.org.