Onions growing in soil that's rich in organic matter don't need supplemental fertilizer, but if yours need a boost of nutrients, you can sidedress with a slow-release, natural fertilizer. Frasier suggests starting with phosphorus- and potassium-rich fertilizers. "Those are important early when the onion is establishing a good root system--an onion has upwards of 200 roots--but after that, what makes a big bulb is a big top." To promote leaf growth, Frasier recommends applying a more nitrogen-heavy fertilizer every few weeks once plants are established. "Each leaf makes a ring [in the onion bulb], and the bigger the leaf, the bigger the ring."
To allow plants to transfer carbohydrates from the leaves to the onion bulbs, discontinue supplementing nitrogen once the bulbing process has begun. At that point, excess nitrogen stimulates leaf growth at the expense of bulb size, Frasier says. "That causes what we call 'thick necks'--where the onion has a thick neck that won't shrivel down properly." Newbie hint: Protect your onion seedlings from fungal diseases by presoaking the seeds, sets, or roots in compost tea before planting. Master's tip: Have cutworms or onion maggots ruined your crop in the past? Apply parasitic nematodes, available in garden centers and in catalogs, to the soil at least a week before planting.
Curing and Storing
When you notice that the tops of most of your onions have begun to fall over, stop watering and mark the calendar. In about two weeks, dig up all of the onions and spread them out in the sun for several hours to dry. Be careful not to bruise the bulbs as you are digging them, or they are likely to rot in storage. You can braid the onion tops while they are still flexible, secure with twine, and hang in a warm, dark, well-ventilated spot to finish drying. Or, you can snip off the onion tops, leaving at least an inch of neck intact, and spread them on a drying rack away from sunlight and moisture to finish the drying process. Reposition each bulb from time to time, so that each dries thoroughly. Brush off, but don't wash, any soil still clinging to the bulb.
Place cured and dried onions in mesh bags, baskets, or ventilated crates, and store them in a dry area where the temperature stays between 30- and 50-F. Check them periodically; if you see sprouts or roots forming, the temperature is too high and conditions too moist. If you keep your storage onions cool and dry, they will keep for up to a year--or just about the time you are ready to harvest the next crop. Onion bulbs form based on the amount of daylight they get. Northern gardeners will succeed with long-day varieties; southerners need to go with short-day options. Everyone else, take your pick of the intermediates.
Newbie hint: Protect your onion seedlings from fungal diseases by presoaking the seeds, sets, or roots in compost tea before planting.
Master's tip: Have cutworms or onion maggots ruined your crop in the past? Apply parasitic nematodes, available in garden centers and in catalogs, to the soil at least a week before planting.