Why Are My Rhubarb Stalks Thin and Spindly?

Rhubarb that produces stalks no thicker than a pencil, when you expected fat, substantial stems, is a plant that is either under stress or past its productive prime. Rhubarb has the capacity to produce impressively thick stalks — 1.5 to 2cm or more in diameter — when it is well-nourished, adequately watered and not overcrowded. Persistent thin stalks across several seasons signal that something in the growing conditions needs to change.

Overcrowded, undivided crowns

A rhubarb crown that has not been divided for more than five years will typically begin producing progressively smaller, thinner stalks as the crown becomes congested. Multiple growing points compete for the same root resource, and none of them gets enough to develop fully. Dividing the crown in autumn or early spring, replanting the strongest sections with good spacing (90cm apart), and enriching the planting hole with compost usually transforms stalk quality within one to two seasons.

Poor soil nutrition

Rhubarb is a heavy feeder. It performs best in deep, fertile soil with plenty of organic matter. An annual top-dressing of well-rotted manure or garden compost applied to the surface around the crown in late winter (not covering the buds) provides the nutrients needed for thick stem development. Plants in light, sandy or depleted soil may also benefit from a granular balanced fertiliser in early spring.

Shade and over-harvesting

Rhubarb needs at least six hours of direct sun to produce strong growth. Plants shaded by neighbouring trees or structures will consistently produce weaker, thinner stems. Over-harvesting — removing too many stems too late in the season — weakens the crown by reducing its photosynthetic capacity going into autumn. Never harvest past midsummer and always leave at least three to four strong stems on the plant at all times.