Onion seeds should be started indoors (with the exception of the South) 10 to 12 weeks ahead of your average last spring frost, and transplanted out 4 to 6 weeks before your average last spring frost. Leeks and shallots also follow the onion rule- the bigger the transplant, the bigger the potential yield, so start these early (8 to 10 weeks before average last spring frost). Shallots are cold hardy and can also be transplanted out in the fall, and over-wintered from Alaska to Hawaii to S. Florida.
Growing onions from seed versus starter plants offers a wider variety, is less expensive, and gives you more control over growing conditions and inputs like fertilizer or pesticides. Plus, we are all itching to get our hands dirty again!
Tips for growing onions, leeks, and shallots
- The ideal soil temperature range
for best germination is 60°–85°F;
although, they will germinate at
45°F, but it will take more time.
- While growing indoors, the green
tops can be trimmed to 3” to keep
them upright and manageable. The
trimmings are similar to chives and
can be used in the same way.
- Thinning while growing indoors is
not necessary. Divide onions to
single plants at transplanting. Sow
bunching onions in groups to save
time and space; they do not require
dividing at transplant time.
- Onions are heavy feeders. Amend
your planting area according to soil
test suggestions. Nitrogen is usually
the first major nutrient to be used
up, so usually a test suggests adding
nitrogen and often, organic material
(compost or ground leaves work well).
- After hardening off, transplant
onions 4” deep. This may leave only a
tiny bit of green poking up through
the soil, but don’t worry, leaves
will quickly catch up. Scallions and
bunching onions can be grown densely;
all others should be separated into
individual plants and transplanted
- Once greens are 8”–10” tall, beds
can be heavily mulched or hilled with
soil to reduce weed pressure,
conserve water, and in the case of
leeks, blanch the stems (keep white).
Leeks should be hilled monthly to
keep as much of the stalk white as
- Keep weeds in check, onions do not
compete well with weeds and will be
- With the exception of
bulb-producing onions and shallots,
the other onions discussed can be
harvested whenever you like. Bunching
onions and leeks are frost tolerant
but should be harvested before a hard
freeze. Many gardeners prefer to
harvest leeks after a light frost,
because plants produce sugars to
avoid freezing, making them sweeter.
Bulbing onions require special attention at sowing because their growth is triggered by day length (latitude). Understanding what varieties grow best in your area is the first step to success.
- Long-day varieties grow well in the
north (above the 37th parallel), as
they need 14 to 16 hours of daylight
to trigger bulb formation. Try
Ringmaster, Yellow Sweet Spanish
Utah, and Cipollini Borettana.
- Intermediate-day varieties overlap
the long and short day ranges a bit
and cover the middle of the country
(32nd to 42nd parallel). These
varieties start the bulbing process
when sunlight reaches 12 to 14 hours.
Try Red Amposta.
- Short-day varieties are best sown
in fall in the south (below the 35th
parallel), for a late winter/early
spring harvest. These varieties need
10 to 12 hours of daylight to trigger
bulbing. Depending on soil
temperature, southern gardeners may
choose to sow onions directly into
the garden. In the north, short-day
onions may be grown over the winter
in a greenhouse, or transplanted out
in the spring; this method produces
early but smaller bulbs. Try Yellow
Granex PRR (Vidalia type).
Harvest bulbs when tops fall over,
and have turned yellow or brown. Cure
bulbs in well-ventilated area for 3
to 7 days before cutting off tops and
most of the root (be careful not to
nick the bulb). Farmers generally
cure bulbs in the field if dry
weather is expected.
Once cured, trimmed, and any extra soil is knocked off (do not wash), bulbing onions and shallots are ideally stored in a cool (40°–55°F), dry place. As a general rule, the sweeter the onion, the shorter the storage time, but you can still expect a month or more if cured and stored properly.
Leeks and bunching onions store best with their roots trimmed close to the stalk, placed in a sealed container like a plastic bag, and stored in the refrigerator crisper. Leeks can be quite long, and you may choose to trim the tops. Leeks and bunching onions, fresh from the garden can store for a month, potentially longer.