Onions

Start with the market

Steve’s first question is the same for every crop: tell me what your market wants. In onions, it’s the size grades, firm bulbs with thin, dry necks, and skins that cure to a good colour and stay on. It’s onions that keep for months without rotting or sprouting. And some markets want them hot, and some want them mild. Most of it is settled in the paddock.

Five cured brown onions, all about the same size, firm, with thin dry necks and whole golden-brown skins. — The market pays for the size grades, firm bulbs with thin, dry necks, and skins that cure to a good colour and stay on: onions that keep for months without rotting or sprouting. Most of it is settled in the paddock. An illustration, simplified.

Three onions: one with a thick green neck, one with its skin split and peeling off, and one sprouting a green shoot from its neck. — The market pays for the size grades, firm bulbs with thin, dry necks, and skins that cure to a good colour and stay on: onions that keep for months without rotting or sprouting. Most of it is settled in the paddock. An illustration, simplified.

Every ring was a leaf

Cut an onion from top to bottom and you can see how it’s built. Each fleshy ring is the swollen base of a leaf, running up through the neck into the green leaf above it. The rings all grow from a small, flat stem at the bottom, the same base the roots come from. So every leaf the plant grows before it starts to bulb is a ring it can fill later. A leaf that was never grown is a ring the bulb will never have.

A young onion plant cut in half lengthwise: hollow blue-green leaves fanning up from a narrow neck of nested leaf bases, on a small flat stem with thin white roots below. — An onion is built from its leaves: most rings of the bulb are the swollen base of a leaf grown before it bulbed, and the last few form without a leaf. An illustration, simplified.

The same onion at harvest, cut in half from top to bottom: thick white rings nested inside papery brown skins, running up into the neck, on the same flat stem. — An onion is built from its leaves: most rings of the bulb are the swollen base of a leaf grown before it bulbed, and the last few form without a leaf. An illustration, simplified.

That makes leaf growth the onion’s time to build its engine, the way the canopy is for any crop. Get an even strike from a small seed, then keep the leaves coming, a new one every week or two, for as long as the days allow. It’s the time for nitrogen, and for the minerals that run the engine: manganese and magnesium.

The days decide

An onion doesn’t start its bulb when it’s big enough. It starts when the days get long enough, and each variety has its own length of day. A short-day onion bulbs on the shorter days of a Queensland spring. A long-day onion waits for the long days of a Tasmanian summer. Warm weather hurries it along. Once bulbing starts, the plant grows no more green leaves: its last few form inside the bulb, as rings without a leaf. That’s why the sowing date matters so much. Sow too late, and the days catch the plant small: it bulbs anyway, with too few rings to fill.

Of all the onion’s critical stages, Steve’s onion program calls the turn to bulbing the make-or-break one. The leaves that are up by then are the frame the bulb is built on. What the plant has taken up by then is what the bulb starts with. So the foundation goes in before it: calcium and boron for firm rings and sound skins, and silicon for tougher leaves. Potassium waits for the bulking, as it does in every crop.

A young onion plant, cut away below the soil: slender, tubular blue-green leaves, a narrow neck with no bulb, and a few short white roots. — Every ring was a leaf, so the leaves come first. The days decide when the bulb starts. An illustration, simplified.

The same onion weeks later: more leaves, and its base swelling into a small bulb about twice as wide as its neck. — Every ring was a leaf, so the leaves come first. The days decide when the bulb starts. An illustration, simplified.

Roots that need a hand

An onion’s roots are few, short and shallow, with almost no root hairs, so on its own it can’t reach much of the soil. It leans hard on the fungi that live on its roots. Their fine threads reach well past the roots and bring back phosphorus and zinc, in trade for sugar. Onions are among the crops that need phosphorus most, and they need it close to the roots, and early.

After a long bare fallow, or a crop like canola that doesn’t host those fungi, a young onion crop can stall for weeks. What grew in the paddock before matters as much as what goes on now. That’s the plant’s side of soil health.

An onion’s roots are few, short and shallow, with almost no root hairs. It leans on the fungi on its roots: their fine threads reach well past the roots and bring back phosphorus and zinc, in trade for sugar. After a long bare fallow, or a crop like canola that doesn’t host those fungi, a young onion crop can stall for weeks. An illustration, simplified.

Bulking: the size and the bite

Once the bulb has started, it fills fast, week on week. That’s bulking, and two minerals lead it. Potassium fills the rings and sizes the bulb: the crop has earned it now. Sulfur is the bite. An onion’s heat comes from the sulfur it builds into its rings, so more sulfur can make a hotter onion, most of all where the soil is short of it. Mild, sweet onions are grown on less, and the variety counts for a lot. It’s one place where what the market wants changes the program.

Keep the water steady while they bulk. A dry spell stops them sizing, and they don’t always pick up again where they left off.

A young onion, its bulb swollen fat and round in the soil, its tubular leaves still up and green, its roots below. — Once the bulb has started, it fills fast. Potassium fills the rings and sizes the bulb. Sulfur is the bite: an onion’s heat comes from the sulfur it builds into its rings, so more sulfur can make a hotter onion, and mild, sweet onions are grown on less. An illustration, simplified.

Once the bulb has started, it fills fast. Potassium fills the rings and sizes the bulb. Sulfur is the bite: an onion’s heat comes from the sulfur it builds into its rings, so more sulfur can make a hotter onion, and mild, sweet onions are grown on less. An illustration, simplified.

Tops over, and a finish that keeps

As the bulb finishes, the neck softens and the tops fall over. That’s the onion saying it’s done: once most of a block is down, the skins can set and the crop can come out to cure. Late nitrogen works against all of it. It keeps the tops growing and the neck thick, and a thick neck doesn’t dry and seal, which is where the storage rots get in. So hold the nitrogen at the finish.

A cut-away of a ripe brown onion half out of the soil, its tops fallen over flat and yellowing, its neck thin and drying. — As an onion finishes, its neck softens and the tops fall over. Hold the nitrogen then, and the neck dries and seals. Late nitrogen keeps the tops growing and the neck thick, and a thick neck doesn’t seal: that’s where the storage rots get in. An illustration, simplified.

The same onion with its tops still upright and dark green, and a thick, green, fleshy neck. — As an onion finishes, its neck softens and the tops fall over. Hold the nitrogen then, and the neck dries and seals. Late nitrogen keeps the tops growing and the neck thick, and a thick neck doesn’t seal: that’s where the storage rots get in. An illustration, simplified.

Curing dries the necks and the outer skins until they’re papery and sealed. Good skins come from the whole season: calcium for strength, and copper. Short of copper, the skins come out thin and pale, and don’t stay on.

Garlic, shallots and leeks

The rest of the family follows the same story. Garlic and shallots (the small bulb onions that multiply, not green onions) build their bulbs from leaves too, and day length turns them to bulbing. Garlic also needs a spell of cold after it’s planted, to divide into cloves. Leeks never bulb: they’re grown for the long white bases of their leaves. In the program builder, they all start from the Garlic and leeks family.

A big golden-brown onion bulb half out of red-brown soil, its green leaves upright and its white roots below. — The rest of the onion family follows the same story. Garlic and shallots build their bulbs from leaves too, and the length of the day turns them to bulbing. Garlic also needs a spell of cold after it’s planted, to divide into cloves. Leeks never bulb: they’re grown for the long white bases of their leaves. An illustration, simplified.

A garlic plant at harvest: flat, grey-green leaves, and a round bulb in papery white skins half out of the soil, its cloves showing through. — The rest of the onion family follows the same story. Garlic and shallots build their bulbs from leaves too, and the length of the day turns them to bulbing. Garlic also needs a spell of cold after it’s planted, to divide into cloves. Leeks never bulb: they’re grown for the long white bases of their leaves. An illustration, simplified.

The season at a glance

About 180 days from sowing. The days are a guide: the crop’s stage is the clock. Build yours in the program builder.

1 · Germination and root establishment days 0–20

Clean, fast emergence and the strongest roots you can grow. Phosphorus feeds the root tips.

How you know it’s here The first green loops push through and straighten into the first leaf.

What matters most: Phosphorus Calcium Zinc Boron Silicon
2 · Early vegetative structure days 20–50Sap test

Dense, firm leaf bases. Calcium and boron build the cell walls.

How you know it’s here One to four true leaves, standing up straight.

What matters most: Calcium Boron Zinc
3 · Canopy development days 50–80Sap test

Bank as many healthy green leaves as you can: each one becomes a ring of the bulb.

How you know it’s here Five to seven leaves, and the canopy filling in.

What matters most: Nitrogen Magnesium Manganese Phosphorus Iron
4 · Bulb initiation and cell division days 80–120Sap testCell division

The bulb’s cells are made now, and their number sets how big it can grow. Hold potassium back.

How you know it’s here The base starts to swell. Bulbing has begun once it’s twice the width of the neck.

What matters most: Calcium Manganese Boron Zinc Copper
5 · Bulb filling and maturation days 120–180Sap test

Fill the cells you built. Then finish the skins, and what makes the bulb store and taste right.

How you know it’s here The bulbs swell fast. Later the necks soften and the tops fall over.

What matters most: Potassium Copper Boron Molybdenum Sulfur

Where it fits in a program

Write down what your buyer pays for: the sizes, the skins, how hot or mild, and how long they have to keep.
Build the leaves while the days are short: an even strike, the engine running, and nitrogen early.
Put the phosphorus close and early, and think about what grew in the paddock before.
Have calcium and boron in before bulbing starts.
Save potassium for the bulking, with sulfur to suit your market.
Hold the nitrogen at the finish, let the tops go over, and cure them well.
On each walk, count the leaves on a few marked plants until bulbing starts. Then measure the bulb against the neck: once it’s twice as wide, it’s bulbing. Sap test before it starts, and through the bulking.

“That’s all my programs. So what I’m going to do for potatoes, beans, onions… because all the things are the same.”

— Steve, on his critical stage programs

Oni­ons