Overwintering Onions: Frost Heave, Feeding and Weeds

Why the tops stop

Through the coldest months an onion barely extends a leaf and the color dulls. What it is doing instead is spreading roots, ready to throw out leaf quickly in spring. It looks stalled but it is not, and feeding it because you are worried simply brings on bolting later.

What the crop needs in winter is protection, not growth. There are three things to protect it from: frost heave, weeds, and roots sitting in water.

1. Protect from heave Frost lifting the roots dries the plant out and kills it. The morning after a hard freeze is when you walk the rows and press the lifted ones back.
2. Protect from weeds Onion leaves are thin and low, so they lose the competition for light with winter weeds. Hoeing them off small saves an enormous amount of work in spring.
3. Protect from standing water Roots sitting in water stop growing in spring. Check after rain that the channel at the foot of the bed has not silted up.

Putting heaved plants back

Frost heave is water in the soil freezing into columns, and the force lifts shallowly planted onions out by the roots. A lifted plant dries out and dies. Walk the rows the day after any hard freeze.

1. Pick your days by the forecast The morning after a night below freezing is the one that matters. You do not need to look daily. Using the forecast means a handful of visits over the winter.
2. Wait until the soil thaws Treading on frozen soil cuts the roots. Go out in the afternoon once the sun has softened it, press soil back around the plant and firm it lightly.
3. Note where it keeps happening If the same patch heaves every year, drainage is the cause. Raise that part of the bed next year or cover the surface with plastic mulch.

An inch of husks or straw around the plants evens out the surface temperature and largely stops the columns forming. Under plastic mulch the damage is minimal.

The deadline for feeding matters more than the number of feeds

Feed three times: once in the depths of winter as the roots extend, once as the leaves start moving in late winter, and once at the start of spring as bulbing begins. Scatter a light dressing between the rows or along the shoulder of the bed, not at the base of the plants, and work it in.

The important part is the last date: finish by the start of spring. Feed later than that and the plant puts on leaf while the bulb should be filling, and the neck never closes. A bulb with an open neck cannot dry down properly, so it rots, and it is the first thing to spoil in store.

In mid spring the leaves look pale and you will want to feed. Whether you can resist is what decides how well the crop keeps in summer.

With mulch and without

Black plastic holds warmth and suppresses weeds and heave at the same time, so winter is far less work. The trade-off is that you cannot work fertilizer into the soil, so you either front-load the base dressing or feed liquid through the planting holes. In mild gardens it can also warm the soil too much in spring and bring on bolting.

Without mulch you can feed and hoe in the same pass, which loosens the surface, kills weeds and lets air to the roots. It is more work, but you look at the plants every time, so you notice problems early.

Do not underestimate winter weeds

Winter annual weeds grow while the onions are standing still. Onions have thin leaves and stay low, so they lose the competition for light. Try to pull the weeds in spring and their roots are tangled with the onion roots and you lift both together.

1. Hoe once before midwinter Run a hoe shallowly between the rows. The weeds are still small with shallow roots, so simply skimming the surface kills them.
2. Hoe again in late winter Do it again before it warms up. Skip this one and by spring the weeds are taller than the crop and you cannot see what you are doing.
3. Hand weed close to the plants A hoe cannot get up against the plants, so use your fingers. Hold the soil down as you pull so the onion does not come with it.

In snow and in hard frost

Where snow lies for weeks it works as insulation, and buried leaves are not a problem. The problem comes when it melts and the water sits. Cut a drainage channel at the foot of the bed so meltwater has somewhere to go.

Where there is no snow but there is cold, dry wind, the leaf tips scorch. That is expected, and as long as the growing point is alive the plant recovers in spring. A windbreak net on the windward side reduces the losses further.

Winter watering is not normally needed. Only if a dry spell leaves the surface dusty should you water once, on a mild morning.