Yes, some euphorbias are very slow to bloom. Photo: hortology.co.uk & clipart-library.com, montage: laidbackgardener.blog
Question: I have a collection of succulents and most of them flower every year. But none of my euphorbias bloom. Why not?
Answer: Your lack of success getting euphorbias (Euphorbia spp.) to bloom is probably due to … your choosing the wrong plants!
Among the 2000 or so species of euphorbia, there are some that bloom readily in the average home and others that will probably never bloom at all.
For example, the crown of thorns (E milii and its hybrid E. × lomi) is not hard to flower. In fact, it often blooms several months a year; under really good conditions, it will practically never be without some bloom.
Another that blooms readily is E. lophogona. In fact, it self-seeds in all the nearby pots in my plant room!
And what about the poinsettia, which is not a succulent, but is an euphorbia (E. pulcherimma)? As long as you give it short days in the fall, it will bloom faithfully at Christmas, at least in the northern hemisphere.
The baseball spurge (E. obesa) is another that blooms regularly, even if its flowers are relatively discreet.
Bigger Isn’t Better
I suspect you’re growing some of the candelabra-type euphorbias, such as the African milk tree (E. trigona) and the milk spurge (E. lactea), two of the most popular indoor euphorbias.
Euphorbias that take this form, that of a small tree with an erect trunk and thick branches, are very slow to flower. You’re going to need a lot of patience—and space!—to see these bloom. Not only do they have a treelike shape, but they take on treelike dimensions. Often these species only flower at a very advanced age (20, 40 or 60 years old) or when the plant has reached a very large size -10 to 20 feet/3 to 6 m tall —, pretty much impossible to reach in the average home.
Also, be aware that, according to Wikipedia, the African milk tree (E. trigona) has never been known to bloom … and never is a very, very long time! (This plant is unknown in the wild and may be a hybrid; hybrids are often infertile or fail to bloom.)
Moreover, candelabra euphorbias are not the only candelabra succulents that are slow to flower. Most of the various candelabra cactuses (Cereus, Acanthocereus, Browningia, Lophocereus, etc.) must also be large and fully mature to bloom. The famous saguaro cactus (Carnegia gigantea), with its big, thick arms, for example, has the reputation of not blooming before it reaches 75 years old!
Note too that crested succulents, that is, mutated ones with stems that grow in the form of a cock’s comb, are so severely deformed they are very likely to flower and several euphorbias have precisely this shape. That includes the very popular crested milk spurge (E. lactea cristata). I don’t know that it has every bloomed.
Euphorbias that are strongly variegated (marbled with albino tissue), too, can be very reluctant to bloom.
Best Conditions Possible
Of course, you still need to have the right conditions if you want to see any euphorbia bloom. Even a “euphorbia that blooms readily” needs a lot of light, even full sun, watering when the soil is dry, but not super dry, reasonable temperatures (very few enjoy cold winters), modest fertilization, etc. to be happy and only a “happy euphorbia” will flower.
But since you say that your other succulents are able to bloom, I suspect that you are giving your euphorbia adequate conditions. After all, euphorbias have about the same needs as a typical succulent.
So, my suggestion to you is to go out and buy a euphorbia specifically recognized for its ease of bloom. In fact, save yourself a lot of doubt and effort and buy it in bloom. And I don’t consider that cheating. Part of being a laidback gardener is always taking the easy way out!