The genus Allium includes many of our favourite and easy-to-grow vegetables – leeks, shallots, onions, garlic and chives. This week on Ask a Master Gardener, Susie discusses tips for a harvest that will delight well into next winter.
Onions and other alliums can be started in several different ways. Sets, seedlings or seeds are all options for the home gardener, each with advantages and disadvantages.
Sets are immature bulbs that have been stored over winter. They are easy to plant and result in an early harvest, but the choice of varieties is often limited. Similarly, there may a be a limited selection if young seedlings are purchased at a nursery.
Susie prefers to start her onions from seed, giving her a wide range of edible options. Browse your favourite seed catalogue and choose from red onions (Rossa di Milano), yellow, white or green, mild types for fresh eating (Walla Walla or Ailsa Craig) or storage varieties (Cortland). The list is almost endless.
Planting from seed requires a bit of planning. Alliums are among the earliest vegetables to be sown indoors, starting in mid-February to early March (about 10 weeks before the last average frost date in your area). It is important to use fresh seed as viability is significantly reduced after only a year or two.
Although the tiny seeds can be planted individually in pots, it is easiest to start them in seed trays or similar containers with drainage holes. Use a good quality potting soil, create shallow furrows and pop the seeds in to a depth of 1/2 – 1 cm. Give them a good drink. Find a warm spot, cover with a clear dome to retain moisture, and watch them sprout in a short 4-10 days. Once sprouted, the cover can be removed.
Some gardeners periodically clip the tops of onion seedlings to a height of about 6-8 cm, although they will do just fine without the haircut. Don’t clip until the onion has produced at least 2 leaves and don’t take off more than 50% with each pruning. Only trim back indoor seedlings. Once they’re in the ground, leave them alone.
As with most vegetable garden ventures, the planting out date for onions will depend on the weather but is generally done once the soil can be worked in mid to late April. Seedlings should be hardened off before planting.
To plant straight rows, mark out lines with twine and loosen the soil beneath. Gently lift seedling strips from their trays. They can be teased apart and spaced in the ground depending on the expected size of the mature bulb. Firm the soil around the plants, water and you’re done.
Leeks are a different story! The edible white stalk is the most mild and tender portion of the leek, although the fibrous tops are a great addition to the stockpot. To maximize the length of the white shaft, leeks are often “hilled”, that is, soil is mounded up against the stalk of the plant as it grows. Alternatively, a dowel can be poked into the ground to deeply bury a leek seedling.
Susie has found success by digging out a rectangular block in her vegetable patch to a depth of about 8 inches. The soil is set aside and the leek seedlings are planted in furrows in the bottom of the hole. This soil is used to gradually refill the hole through the season as the leeks mature.
Plant care through the season
Once planting is complete, pest pressures can be reduced by covering seedlings with a floating row cover – a lightweight, semi-transparent fabric that serves as an insect barrier. Use rocks or soil to hold down the edges of the cover. This effectively excludes the nocturnal leek moth, which lays its eggs on the leaves. Once hatched, the moth larvae tunnel or mine into leaves or bulbs of many types of alliums.
Remove weeds to reduce competition for sun and nutrients, and keep the soil moist but not wet. About one inch per week in the absence of rain is ideal. Mulching with straw or other organic matter is a great way to control weeds and retain soil moisture.
Garden myths abound in the age of misinformation (for more, check out our previous posts) and onion growing is no exception. Some advocates believe that breaking the tops of onions will result in larger bulb size. Not true! Bulb size is in part due to the movement of sugars manufactured in the foliage moving to the bulb. Breaking the tops will therefore interfere with, not promote, bulb enlargement.
Day length and onions
Onion bulbs form in response to both temperature and day length. In Canada, where our summer days are long, gardeners should choose “long-day” onions. Most Canadian seed catalogues will offer seeds suited to our day length. If you garden in the southern US on the other hand, “short-day” onions are a better choice.
Harvest and storage
If you can’t wait to taste a delicious home-grown onion, they can be harvested early as green onions. If they are allowed to continue growing, the tops will flop over and begin to dry out as they reach maturity. Stop watering at this point, and when about half the tops have turned brown, they can be carefully eased from the ground.
Leave the newly harvested bulbs in the sun for a few days if the weather stays dry. Then resume curing the bulbs in a well-ventilated location out of the sun for 2-4 weeks or until the tops and necks have thoroughly dried. During this curing process the papery skin will tighten around the bulb and the stems and roots can then be clipped to a length of at least one cm.
Keep storage onions and shallots in a cool dry place with good air circulation for up to 6 months. Sweet onions and leeks do not store well and will need to be eaten or made into prepared foods for the freezer (think leek soup).
Is this the year to give alliums a try?