Plant Fact Sheet

FABACEAE
Umbrella Thorn
Acacia tortilis

This tree is the quintessential symbol of the African flora. With its flattened crown of thorny branches spreading to twice its fifty foot height, Umbrella thorn evokes the image of the African savannah in all who see it. One of 120 acacias native to Africa, Umbrella thorn is easily identified by its uniquely contorted seed pods and distinctive thorns – curved and straight on the same plant. Gardeners at The Living Desert can attest to the pugnacious quality of the thorns and the aptness of the Afrikaans common name ‘haak-en-steek’ – grab and stab. Overall size is dictated by growing conditions and oftentimes this plant may be no more than a 6-8 foot shrub.

The foliage is consumed by browsers. Balls of fragrant, creamy-white flowers occur in late spring to summer and are favored by monkeys and baboons. The nutritious pods, with a content of nearly 20% protein, are highly sought by many animals including antelope, giraffe, elephant, monkeys and baboons. A hard seed coat protects the embryo through the digestion process and the partially scarified but undigested seed is ‘sown’ in a pile of dung, fertilized and ready to germinate.

Like the mesquite (Prosopis sp.) in the American southwest, Acacia tortilis is an encroacher, following man and his beasts as they alter the landscape. Umbrella thorn is often associated with ‘sweet veldt’ or good grazing, during the wet season. In arid lands moisture and therefore organic materials are in limited supply. The breakdown or decay of organic materials supplies plants with a usable form of nitrogen – an essential element for plant growth. Nitrogen as an element is not in short supply. The air we breathe consists of 78% nitrogen gas, but this is not a form that can be directly used by plants. Acacia tortilis, like many members of the legume family Fabaceae have the ability to convert or fix atmospheric nitrogen into a soluble, usable form. Through a mutually beneficial (symbiotic) relationship, bacteria (Rhizobium) actually do the fixing in nodules formed on the plant’s roots. In the enriched environment of available nitrogen surrounding the acacia other plants including sweet grasses are able to prosper. The Living Desert has over 40 taxa of African acacia represented, making it one of the richest collections of these plants outside of Africa.