Leeks left alone stay mostly green, with only a short pale section at the base. Mounding soil against the stem as it grows blocks light from reaching that part, and it stays white and mild instead of turning tough and green. Early September is usually the last worthwhile round before growth slows for winter, so it's worth getting a final few centimetres on now.

Earth Up Leeks for a Longer Blanched Stem

What you need

1 pair gloves soil can carry tetanus spores
1 string line optional, only needed if the row has drifted out of line
2 pegs to anchor the string line

Step by step

Put on gloves, then loosen any trodden or capped soil in the paths either side of the row with a garden fork, taking care not to catch the leek roots.
10 min
Weed along the row by hand — soil drawn up over a weed just buries it rather than killing it, and it will push back through the mound within a week or two.
10 min
If the row has drifted out of line, run a string line along one side pegged at each end, and earth up to that rather than eyeballing it.
5 min
Working from one end, draw soil up from both sides of the row with a hoe, pulling it toward your feet at a shallow angle rather than chopping downward.
15 min
Firm the soil against each stem by hand or with a trowel, stopping where the leaves fan out — soil pushed higher than that traps moisture against the stem and can rot it instead of blanching it.
10 min
Repeat along the row, aiming for an even mound about 8-10cm high rather than a few tall heaps against individual plants.
5 min
Water along the row if the soil is dry, so the mound beds in and settles rather than collapsing at the first heavy rain.
5 min

Afterwards

Rinse soil off the hoe, fork and trowel before putting them away — dried mud pits the metal over time. Check the row again after the next heavy rain; a mound that has washed flat needs firming back up by hand, and any stem that has been knocked over should be pressed upright before more soil goes around it.

Questions

How many times should I earth up in a season?

Two or three times, a few weeks apart, adding a few centimetres of soil each time rather than trying to bank it all up at once. Doing it gradually gives the stem time to keep growing before it's covered again.

What if I don't earth up at all?

The leeks are still perfectly edible — the whole stem just stays more green and slightly tougher, since it hasn't been blocked from the light.

Is it too late to earth up in September?

Not for a final round. Growth slows once night temperatures drop, so this is usually the last worthwhile pass before winter rather than the start of a new one.

Can I use compost instead of garden soil?

Loose garden soil packs and sheds rain better around the stem. Compost stays wetter against the plant and is more likely to encourage rot at the growing point.