Boron deficiency

Hollow brown stems in broccoli, black heart in beetroot, corky cracked apples — a shortage of a nutrient needed in tiny amounts and easy to overdose.

Disorder Serious Doesn't spread

What you'll see

Brown, hollow, corky centers in broccoli, cauliflower and swede stems and roots.
Black heart in beetroot: a hard black rotted core in an otherwise sound root.
Growing points dying, so the plant makes a rosette of side shoots instead of a leader.
Cracked, corky, misshapen apples and pears with brown flecks in the flesh.
Celery stems cracked lengthways with brown streaks, and cauliflower curds browned and bitter.

Why it happens

Boron is needed in minute quantities to build cell walls and to move sugars, and it does not move around inside the plant — so a shortage always shows first in the newest tissue and the growing point. It leaches easily from light soils, and becomes unavailable in alkaline soil and in drought, because the plant takes it up in soil water.

Light sandy soils, heavily limed soils above pH 7, drought, and high-boron-demand crops: brassicas, beetroot, celery and apples. Overliming is one of the most common triggers of all.

Treating it

Test before treating. Boron is toxic at not much above the useful rate, and an overdose is far worse than the deficiency.
Apply borax at the rate on the packet and no more — typically a level tablespoon dissolved in a watering can for a 10-square-meter bed, once only.
Water it on evenly in solution rather than scattering granules, which creates toxic hotspots.
Foliar-feed with a very dilute solution for a faster correction on a standing crop.
Water through drought; boron already in the soil is often simply unreachable in dry ground.

Keeping it away

Add compost and well-rotted manure annually; organic matter is the main reservoir of boron in garden soil.
Do not over-lime. Keep brassica beds at pH 6.5 to 7.0 rather than pushing higher against clubroot.
Water consistently in drought, especially on light soil.
Grow a green manure over winter on sandy soil to hold what is there.
Test the soil every few years rather than adding boron on suspicion.

Who gets it

Broccoli, cauliflower, swede, turnip, beetroot, celery and apples above all.