Issue: June 3, 2006
My mother would like your recommendation for a fertilizer for a white bunching onion (a Japanese Allium species called Rakkyo), something like a cross between a shallot and a garlic. We plant it in boxes and have gotten good crops just using some steer manure, but we were wondering what you would recommend for the development of large bulbs.Ê Also, what time of year should we plant it and when should we harvest it? One year we waited until flower stalks had formed. This year Mom insisted on harvesting the bulbs right away (a few days ago) and says we should re-plant the large bulbs in about a week. I thought we should wait until the leaves turned yellow.
Although New Mexico is an important commercial producer of large-bulb onions, bunching onions are a good choice for home gardeners. These onions do not produce large bulbs but are useful because you can harvest a few "green" onions (small bulbs and leaves) as you need them while they are growing. You may also harvest only a few leaves for flavoring a salad, soup, or other dish. This makes them very convenient for the kitchen gardener.
Store-bought green onions are often large-bulb onions harvested before the bulb enlarged. Rakkyo onions will not produce the large bulbs. It is sometimes called Japanese chives. Unlike true chives, the bulb will enlarge somewhat (to shallot size) but will not reach the size of the large bulb onions you purchase in the grocery store. If you want to grow large-bulb onions, you should purchase the onion sets (transplants) that will produce the larger bulbs. Grow them in addition to the bunching onions. You described the Rakkyo as a cross between a shallot (mild flavored onion) and garlic. Its flavor is unique. There are other shallots, chives, and bunching onions to add to the kitchen garden collection along with the large-bulb onions. Each type will have its distinct flavor and contribution to culinary art in the kitchen.
You can leave the bunching onions in the ground while they are dormant in the summer, or you can dig them and store them to replant as the weather begins to cool (August or later). The small bulbs of the bunching onions will best survive storage if you wait until the foliage yellows before digging them. If you dug them while still green, plant them more quickly. However, you have probably waited long enough if you just dug the bulbs recently. The leaves should stay attached until they dry, and then you can remove them.
Research at New Mexico State University has shown that the primary fertilizer nutrients needed by onions in New Mexico are nitrogen (which the steer manure provides) and phosphate. Our soils have high levels of phosphorous, but it is often not available to plants because of the high calcium content or our soils. To best determine how to fertilize your onions, have your soil tested. You can get soil testing information from your local Cooperative Extension Service office. Tell the testing laboratory what crop you plan to grow.
Research has also shown that crop rotation is important when growing onions. It is best if onions are not grown in the same soil more often than once every 5 years. Since you are growing the plants in boxes and to prevent accumulation of disease organisms in the soil, change the soil in the box every year, if possible. You can also grow many types of onions in native New Mexico soil after preparing the soil according to directions from the soil testing laboratory (unless you live in an apartment and have no space to garden except in pots and boxes).
Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: firstname.lastname@example.org, office: 505-865-7340, ext. 113.
Please copy your County Extension Agent and indicate your county of residence when you submit your question!