April 15, 2010 at 9:45 a.m.
All onions like direct sunlight with soil that has been well prepared with good drainage. I realize that many gardeners, especially those with raised beds plant their onions closer together, but my rows are forty-two inches wide. I dig a trench about three inches deep and put water in the trench. After the water has settled I plant my sets in the mud (four inches apart if they are Spanish Yellow.) I cover the bulbs with loose dirt but don't pack it down.
Onions like most plants need adequate water so they will make large bulbs but too much can cause foliage diseases. Continue watering until the necks start falling over and the onions mature, and then discontinue so the soil can dry out.
After the roots have been established they may need some fertilizer. Since I have alkaline soil, meaning my soil is at least a 6.5 pH, I use a sulfur-based nitrogen such as ammonium sulfur (21-0-0). If your soil is acidic (below 6.5 pH), a calcium-based fertilizer such as calcium nitrate will provide nitrogen while raising the pH. I apply fertilizer a couple of times during the summer depending on the weather but don't apply any the last month before harvest.
Weeds are the enemy of almost every plant including onions. As I weed I move the dirt away from the bulbs. By the end of the summer most of the bulb should be above ground. When gardeners are concerned about their onions it could be because there was too much dirt over them so they didn't get enough sun or they couldn't expand. Also weeds rob plants of nutrients and moisture.
The two major diseases that will affect onions are blight and purple blotch. The first symptoms begin as small white spots surrounded by a greenish halo. Eventually, leaf death results and bulbs from infected plants may be small because growth is reduced by leaf loss. Usually good air movement and weed control will be sufficient.
Insects causing the most damage to onions are thrips. They are light brown in color and approximately one millimeter long. They feed by sucking the juices of the leaves causing deformed plants with silvery blotches. Thrips over-winter in weeds so reduce pest population by keeping the garden clean. If you have a severe infestation treat with Neem Oil.
As I mentioned earlier, don't harvest your onions until the stalk has bent completely over and turned brown and dry. Bending the top over early only reduces the size of the onion. Some gardeners don't pick them until they must. I watch the weather and pull them during a dry period and leave them on top of the ground for a day or two. Spread them out in the garage or shed until there is danger of them freezing, and then I remove them to the basement.
The next class in our Spring Series will be held Saturday, April 17 at the Senior Center in North Branch. Dianne Patras and Joan Chouinared, Chisago County Master Gardeners will share their years of gardening experience. The class is from 9 a.m. until noon and the cost is $5 per person. Call the Extension Office for more information 213-8901.