The red African milk tree (Euphorbia trigona ‘Rubra’) is often burgundy red when you purchase it, then turns green under the lower light of your home. Source: http://www.ebay.com

Question: I bought a euphorbia as a houseplant. I chose it because it was dark red, but now the new shoots are green and it’s losing its red coloration. Does it need more sunlight? And how do I water it?

Nicole

Answer: The euphorbia you bought is probably Euphorbia trigona ‘Rubra’ (also sold as E. trigona rubra or E. trigona ‘Royal Red’), often called the red African milk tree because of its milky white sap and African origin. It’s a mutation of the normal form that has triangular green stems marbled with white and small green leaves. Exposed to intense sun, ‘Rubra’ produces a reddish stem and red leaves. This coloration tends to disappear in winter, even in summer when the plant is in too much shade. If it’s put back into the sun, the reddish color returns.

20181209C www.bonsaiempire.nl.jpg
Many other succulents redden in bright light, like the jade plant (Crassula ovata). Source: http://www.bonsaiempire.nl
20181209W Echeveria 'Black Prince' www.provenwinners.com & www.tissuecultureaustralia.com.jpg
Echeveria  “Black Prince” in summer (left) and winter (right). Source: http://www.tissuecultureaustralia.com.au & http://www.provenwinners.com

This euphorbia is not the only plant that reacts that way. Many other plants, especially succulents (aloes, echeverias, crassulas, rhipsalis, etc.), turn reddish in full sun, because the red pigmentation, caused by a buildup of anthocyanins, acts as a kind of sunscreen, protecting the stems and leaves against the harmful effects of the sun and especially its ultraviolet rays. Under lower light, the “sunscreen” is not needed and fades away, leaving a green plant.

 

20181209X mikesgardentop5plants.wordpress.com.jpg
Pieris japonica ‘Mountain Fire’. The new leaves are bright red, but will turn green when they mature. Source: mikesgardentop5plants.wordpress.com

In many non-succulents, there is a similar situation, except it’s the fragile new leaves that are red at first, but then become a normal green color as they mature and harden off.

What to Do?

20181209B absolutely_fuzz, reddit.com.jpg
This Euphorbia trigona ‘Rubra’ was red at first, started to turn green, then reddened up again when exposed to more intense light. Source: absolutely_fuzz, reddit.com

It is clear that your euphorbia is not getting enough sun for it to maintain its reddish color. Before you bought it, it was probably grown in a greenhouse, where providing intense sunshine is easy, but in the home, it really should be placed near a sunny window at all times. This is especially true in the winter, when the sun is much, much less intense. And for an even more intense coloring, grow it outdoors in full sun during the summer months.

As for watering, like most succulent euphorbias, the African milk tree is very tolerant of irregular watering. Ideally, you’d water abundantly, then allow the growing mix to dry thoroughly before watering again. The frequency of watering will vary according to the conditions and the seasons: you may need to water it weekly in hot summer weather, but only every two to three weeks under cooler, shadier winter conditions.

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

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