Sorrel can grow to a couple of feet in diameter and develops a heavy root system, which makes it hard to dig up or divide. Leaves of established clumps also tend to be slightly coarser and pack more of an acidic punch. You wouldn't be alone if you preferred to grow it as an annual when the leaves are softer and more suitable for salads. Sow seed from March to May, and thin seedlings to around 4 inches (10 cms) apart, using the baby leaves for salad.