Why Is the Inside of My Beetroot Black and Rotten?

Cutting into a beetroot and finding black, grey, or brown internal tissue — dry and corky, or wet and collapsed, concentrated in the centre or spreading through the root — is a symptom of boron deficiency, a condition called heart rot. The outer root can appear perfectly normal while the tissue inside has broken down or failed to develop properly. Heart rot occurs when beetroot is unable to absorb enough boron from the soil, usually because the soil pH is too high, the soil has been over-limed, or because dry conditions have reduced boron availability. Boron is an essential micronutrient that beets require in relatively large amounts compared to other vegetables.

Why boron deficiency causes internal rot

Boron is essential for cell wall formation and cell division in meristematic (actively dividing) tissue. In beetroot, the central growing tissue — the vascular tissue and surrounding cells — is most actively dividing during root development and is therefore the first tissue to suffer when boron supply is insufficient. The cells fail to form correctly, die, and turn black or brown, creating the characteristic internal rotting or blackening. External leaf symptoms of boron deficiency include distorted, thickened young leaves and the growing tip dying back, but these often appear after internal root damage is already significant.

Causes of boron deficiency in beetroot

High soil pH (above 7.0) strongly limits boron availability because boron binds to soil particles in alkaline conditions and becomes unavailable for plant uptake. Soils that have been heavily limed frequently become boron-deficient for beetroot. Dry growing conditions also reduce boron uptake because boron moves to roots primarily through mass flow in water — drought interrupts this. Sandy, free-draining soils are naturally low in boron. Growing beetroot in containers where the compost has been repeatedly flushed also depletes available boron rapidly.

Treating and preventing heart rot

For the current season: water in a foliar or soil drench of borax dissolved in water — a very small amount (20 g per 10 litres per 10 m²) applied once is usually sufficient; boron is toxic to plants at high concentrations so accuracy matters. For future crops: correct soil pH to 6.5–7.0 if it is above 7.0; apply a general trace element fertiliser containing boron before sowing; maintain consistent soil moisture through the growing season. In containers, use a quality potting compost and ensure it does not dry out severely during the growing period.