Corking means a change in the outer layer of skin (epidermis) of the plant. Most commonly this is a brown colour accompanied by a thickening of the tissues. The cause can be strong sunshine on previously shaded plants, but corking is also a natural phenomenon in habitat with aging plants. Although we may think corking is unattractive, it is a plant’s natural reaction and needs no treatment.

Normal corking with the age
Euphorbia excelsa	Euphorbia mammillaris	Euphorbia polyacantha