You can grow leeks in one of two ways – either as an early spring crop similar to onions or as a late season crop by direct sowing into the garden.
When to Plant
For the early crop, sow seed in pots or flats as thinly as possible. Do this six to eight weeks before outdoor transplanting. This plant shrugs off cold soils and late frosts so planting in late April is common in zone 5.
If you want a late crop, dig a 15 cm. deep furrow (see below for growing details) and plant the seed directly into the furrow in the first week of September
Where to Plant
Full sun. Note that leeks like rich, fertile soil so adding compost is necessary if you want a great crop. They also do not like being water starved.
How to Plant
Sow seed .6 cm deep, cover the seed and gently firm the soil. Keep the soil temperature at 18-21C for indoor plantings; you should see a germination rate of 75% within two to three weeks. This seed does not germinate as quickly nor does it germinate as completely as onions. If you have the ability to cool the seeds down at night, you’ll find they germinate a little bit better. So day germination temperatures around 21C and night temperatures around 15.5C will give you optimum germination.
As soon as you can get onto the garden (late April) transplant them outside. The way to do this is to dig a 15 cm deep furrow and plant the leeks at the bottom of this furrow or trench. Over the course of the growing season, the trench is gradually filled in when you cultivate near the leek to blanche the leek. Seedlings should be planted approximately 10-15 cm apart. Thin to 15 cm when the crop gets really growing.
Care & Maintenance
There are two schools of thought about the tops when you start your own plants. The first says to trim the tops off – cut the leek halfway back – when it gets to 10-15 cm tall. The thinking is that this bulks up the root and makes transplanting better. This is the time-honoured method. A second school says not to cut back the tops as this retards the performance in the garden and this has been suggested by at least one research study.
Whichever you pick (I never cut them back after I read the article) after they germinate, you want to grow them at 10C to keep them short and blocky. Higher temperatures will indeed cause them to get too tall and perform badly in the garden.
Leeks are dug in October before a really hard frost softens them. They are used fresh although storage in frost free conditions will keep them for a long time.
If you keep the frost away from them and bank them up with straw that they’ll start growing again the following April which will make them very early to harvest and eat.
(all numbers rounded out)
1/4 inch = .6 cm
1/2 inch = 1.3 cm
1 inch = 2.5 cm
6 inch = 15 cm
12 inch = 30 cm
18 inch = 45 cm
36 inch = 91 cm