Boron · The wiki · Learning hub

What it does

Boron is the second of Steve’s three amigos. Calcium is the glue between cells; boron ties the cell walls together, linking the long chains the walls are made of so they hold their shape as they grow. Short of it, walls come out weak, and the parts of the plant that are building new walls fastest suffer first: the tips of shoots and roots, and the flowers.

A young bean plant, cut away below the ground: trifoliate leaves, all an even green, with a few small white flowers, and its roots in red-brown soil. — Boron ties the cell walls together, so they hold their shape as they grow. Short of it, the walls come out weak, and the parts of the plant building new walls fastest suffer first: the tips of the shoots and the roots, and the flowers. An illustration, simplified.

Boron ties the cell walls together, so they hold their shape as they grow. Short of it, the walls come out weak, and the parts of the plant building new walls fastest suffer first: the tips of the shoots and the roots, and the flowers. An illustration, simplified.

Boron keeps the sugars moving, too. The leaves make sugar all day, and it has to travel out to wherever the plant is growing: the roots, the new shoots, the flowers and the fruit. Short of boron, the leaves still make it, but it doesn’t get away. It piles up in the leaves as sugar and starch. The leaves then slow their sugar-making to match. The growing tips and the fruit go short.

From flower to fruit

This is where boron earns its keep. When a grain of pollen lands on a flower, it has to grow a tube all the way down to where the seed will form, and that tube only grows at its tip. The tip is building new wall as fast as it can, and it needs boron to do it. Short of boron, the tube stalls or bursts before it gets there.

An almond blossom cut in half lengthwise: pink-white petals, a ring of stamens round a small cup, and in its centre one pistil, a long slender style rising from an ovary holding a single seed. — Pollen grows a tube down to where the seed will form, building it at the tip as it goes, and that tip needs boron. Short of it, the tube stalls or bursts, and the fruit doesn’t set. An illustration, simplified.

No tube, no seed. And in a lot of fruit, too few seeds means small, lopsided or dropped fruit. So a boron shortage often shows up as poor set: plenty of flowers, not much fruit. In other crops it shows in the stems and the heart: hollow stems in broccoli and cauliflower, cracked stems in celery, heart rot in beetroot.

A red apple cut across its middle: a five-pointed star of chambers in the centre, two dark seeds in each, and even, creamy flesh all round. — No tube, no seed. And in a lot of fruit, too few seeds means small, lopsided or dropped fruit. So a boron shortage often shows up as poor set: plenty of flowers, not much fruit. An illustration, simplified.

Another apple, lopsided: its star sits off centre, with seeds only in the chambers on its fuller side and none on the flattened side. — No tube, no seed. And in a lot of fruit, too few seeds means small, lopsided or dropped fruit. So a boron shortage often shows up as poor set: plenty of flowers, not much fruit. An illustration, simplified.

How it moves

Boron rides the water, like calcium, and in most crops it stays where it lands. (A few, like apples, pears and stone fruit, can move some of it around in their sap.) So the plant can’t borrow boron from its old leaves to feed new growth. When it runs short, the growing points and the flowers go without first.

That also means it has to keep coming. The plant needs only a little boron, but it needs it steadily, all the way through the stages where it’s making new tissue.

A young bean plant: dark green older leaves low on the stem, lighter new leaves above, and buds and white flowers at the top. — Boron rides the water, and in most crops it stays where it lands, so the plant can’t borrow it from its old leaves for its new growth. When it runs short, the growing points and the flowers go without first. So it has to keep coming, a little, steadily. An illustration, simplified.

The same bean plant, its older leaves still green, but its tip dead and black, its newest leaves small and crumpled, and its flower buds dropped on the ground. — Boron rides the water, and in most crops it stays where it lands, so the plant can’t borrow it from its old leaves for its new growth. When it runs short, the growing points and the flowers go without first. So it has to keep coming, a little, steadily. An illustration, simplified.

The finest line

Here’s the catch. The gap between not enough boron and too much is narrower than for any other nutrient. Too much burns: the edges and tips of the older leaves go yellow, then brown, because that’s where the water ends its trip and leaves its boron behind.

Three young bean plants in a row: one short of boron, its growing tip dead and its new leaves misshapen; one healthy and in flower; one given too much, its older leaves yellow and brown at the edges. — Too little boron and the tips fail; too much and the older leaves’ edges burn. The window between is narrow. An illustration, simplified.

So with boron you never guess. Test before you add it, keep the amounts small, and don’t reach for more because a little helped. Sandy soils lose it in a wet season; dry topsoil and high pH hold it back. Your tests will tell you which way you’re heading.

When it matters most, crop by crop

The stages where boron is one of the minerals that matter most, in the program builder’s seasons. Each bar is a whole season, from sowing or planting to harvest, or a tree’s year. Build yours in the program builder.

Potatoes about 120 days

Sprout development · Stolon development · Tuber initiation (hooking) · Tuber bulking · Maturation
Beans about 70 days

Vegetative growth · Bud development · Flowering · Cell division · Maturity
Carrots about 120 days

Root initiation · Early bulking
Onions about 180 days

Germination and root establishment · Early vegetative structure · Bulb initiation and cell division · Bulb filling and maturation
Poppies about 140 days

Stem elongation · Hook to flowering
Sweet corn about 90 days

Cob formation · Tasselling and silking
Cotton about 180 days

Squaring · Flowering and boll set
Wheat about 170 days

Stem elongation · Heading and flowering
Avocados a year

Winter: flower buds · Flowering · Fruit set and cell division · Harvest and recovery
Apples a year

Leaf drop and winter wash · Deep dormancy · Wood wash before bud break · Silver tip and bud swell · Green tip to half-inch green · Full bloom · Petal fall · Fruit set, 6 to 10 mm · Fruit sizing · Colour and ripening

Where it fits in a program

Boron goes in with the foundation. Steve pours the three amigos (calcium, boron and silicon) first, to build the cells’ structure. Potassium fills the house after. Boron is needed wherever the plant is making new tissue fast: in early growth, and again at flowering and fruit set, where every pollen tube needs it to grow.

Test first. A sap or leaf test tells you whether you’re short, and whether you’re already close to too much.
Time it before flowering. By the time poor set shows, the flowers are gone.
Little and often beats a lot at once: it keeps the supply steady and keeps you clear of the burn.
Keep the water moving. Boron rides the water, so a dry, stressed plant isn’t moving it anywhere.
Rates come from your tests, your agronomist and the label. Never a guess.

“Calcium, boron and silicon, which I call the three amigos.”

— Steve, on the foundation minerals