by Bruce Brethauer
Euphorbia knuthii is one of those plants which earns its reputation of being an easy plant (it is practically indestructible), with seasonally rapid growth, and for producing remarkable, tuberous roots, which in many plants in cultivation are raised above the soil line so that these can be seen and more readily appreciated. Its thin stems are square in cross section, with two-toned variegation; the tubercles and "ribs" being a darker color, with other portions a lighter green. Plants grown under very bright light may tend to produce additional reddish pigments, and these plants may have a more grey to reddish green coloration to its stems. The stems branch from the tuberous base of the plant, and initially grow upright, but in time, as the stems become increasingly long, and too heavy to support their own weight, these eventually become more scrambling. Short spines are produced in pairs. The spines and spine shields are initially a coppery brown, but will eventually age to a more grayish coloration. This is one of a number Euphorbias which produce a caudex of large tuberous roots - in plants which have been raised from cuttings, these roots tend to be thinner (like branching, contorted carrots), and are more branched, eventually producing huge masses of many roots. In seed raised plants, the roots tend to be much larger (more yam-like in size and shape) and tend to be less branching. While the stems of plants in cultivation are seldom deciduous, it can be argued that this species is geophytic - with the majority of the plant (the tuberous roots) occurring underground, and (under conditions of extreme drought) the stems being shed to conserve water. As with most other Euphorbias, this species produces small greenish flowers (cyathia) which (when pollinated) will produce small capsules, each containing 3 seed. These capsules burst upon ripening, dispersing the seed, sometimes to distances exceeding 10 feet.
Over the years, the members of the various Ohio chapters of the Cactus and Succulent Society of America have traded cuttings of Euphorbia knuthii to such an extant that we have come to regard the plant as a ubiquitous, almost weedy species, and we hardly give it a second look anymore. This is a shame, because this species really does deserve a more prominent place in our collections. If the plant were rare in habitat, or difficult to maintain in cultivation, I’m sure that it would receive considerably more interest. But it may actually be too easy to grow to garner much interest from the succulent aficionados - it tolerates practically any conditions, surviving extended neglect, relatively low light, and other indignities. Even when provided with conditions which are considerably less than optimal, this plant will grow, and may even seem to be thriving: but it should be pointed out that while this is an easy plant to grow, it is even easier to grow it badly. Poorly grown plants will tend to produce fewer, and badly etiolated growth (weak, gangly stems with unusually long internodes, and generally paler green coloration). I suspect that it is on the basis of such poorly grown plants that many of us have come to regard this species as something of an ugly duckling. Many growers have complained that its rambling stems catch on other plants in their collections, or may grapple the clothing of persons who pass too closely to their plants. More than a few persons have tossed their plants, or given them away for this reason alone. But it is possible to keep the growth of this species compact, with denser, shorter stems of a more uniform length - and rather than looking weedy, such plants can be outright attractive.
I recommend my basic succulent growing instructions, with a few modifications. As with most euphorbias, Euphorbia knuthii seems to grow best under very bright light and warm temperatures - I prefer to move my plant outdoors in the summer months to benefit from direct exposure to light, and especially exposure to high summer temperatures. Even though it is extremely drought tolerant, at this time of year, I try to keep its soil reasonably moist, watering it about once every week, and fertilizing it approximately once a month. During this period, this species should produce considerable growth, with each stem adding at least several inches of growth. My plant usually produces its cyathia ("flowers") in mid to late summer - and while the flowers are small and are not brightly colored, they do add a bit of interest - and for those persons who are interested in growing this plant from seed, it would do them well to monitor their plants at this time to harvest the seed when they are ripe (more on this later). Unlike most other tuberous rooted species of Euphorbia, at this time of year, this species will also tolerate lots of water, so if you are experiencing heavier rainfall than normal, there is little need to protect these plants from the additional rain. I have grown several plants over the years, and I cannot recall losing a plant from root rot from excessive water. Where I depart from my generic growing instructions is in the winter treatment of this species: during winter, you want to prevent this species from growing at all. I suggest keeping it even drier than I usually suggest, watering it no more than once every 4 to 6 weeks through its winter dormancy - especially in late winter. Keep the plant cooler at this time of year, but maintain the light as bright as you can so that any growth which it may produce at this time will not be etiolated. Every year or two, near the end of winter, or in very early spring, prune off virtually all of the stems, cutting them back to the soil line (or to the roots if you grow your plants with the roots partially exposed). If you are a bit squeamish about this extreme treatment, you may maintain one or two of the very shortest stems - but otherwise, cut them all. As with all of the Euphorbias, E. knuthii also produces a irritating sap which may cause skin rashes or even blistering where it comes into contact with he skin, so try to avoid all contact with the sap, and wash you hands thoroughly after pruning the plant. After this pruning, allow the plant to heal at least a week to ten days before watering it. As the days increase in length and temperatures rise in spring, gradually increase the frequency of watering - by late spring or early summer, the plant should be producing new growth. The heavy pruning encourages the growth of many additional branches, and keeps the length of each branch comparatively short, and uniform. This pruning will also eliminate any etiolated growth from winter growth, or as a result of bad growing practices - so unlike other succulent plants, the mistakes of the past do not necessarily have the permanently scar this plant for life.
As soon as any danger of frosts has passed, move the plant outdoors, gradually increasing its exposure to daylight over a period of about 10 days to 2 weeks.
While most of the plants which I have seen in cultivation have been raised from cuttings, and therefore produce roots similar to those featured here, seed raised plants will tend to produce tubers which are larger, and less branched. The roots on my plant, which I raised from seed, looked like a forked sweet potato at the time when I last repotted it about a year ago. I estimate this plant to be about 7 or 8 years old, and I suspect that I could have grown it to this size in as little as 3 or 4 years under optimal growing conditions. I have resisted the temptation to expose the roots so far, as the exposed portions of the roots will no longer increase in size once they have been lifted above the soil line. Most growers will gradually expose the roots of their plants, raising the roots slightly with each repotting - in very old plants, the roots can achieve massive proportions - So I recommend keeping a plant for many years to grow unusually large roots.
While it is extremely easy to grow this plant from cuttings, I recommend that you at least attempt to grow a plant from seed should your plant produce cyathia. Grown outdoors, a good portion of the "flowers" of my plant will set seed; the local insect population manages to pollinate the flowers well enough, but you can assist by transferring pollen from the anthers, to the stigma lobes with a fine artists brush (or, according to Mike Massara, of Out of Africa Nursery), the shed whiskers of a cat makes an even better "brush" for this purpose - as these are better at lifting the pollen from the anthers. Following successful pollination, the cyathia produce fruiting bodies consisting of 3 chambered capsules containing a total of 3 seed. When the seed are ripe, the capsules "explode", scattering the seed many feet. It is vital to collect the seed when they are ripe, if the seed are harvested too early, germination rates are negligible, but wait too long, and the capsule bursts, and the seed are too scattered to collect. Mike suggests giving the mature capsule a small pinch or scratch - if the wound exudes a white sap, the seed are not mature, but if the capsule does not exude sap, the capsule can be harvested, and moved into a dry, closed container such as a jar: when the capsule bursts, and releases its seed, the seed can be easily gathered. Let the seed dry in the open air for a few days to dry completely (left in the sealed "collection jar", and they will tend to become moldy, and will loose all viability.
The seed of most of the succulent Euphorbias will loose viability in storage, and may not be viable the following year, so the seed should be planted as soon after harvesting as possible. I suggest a good seed starting mix, with a little grit added to improve the drainage. Barely cover the seed with soil (or grit), and keep the soil warm and evenly moist. Germination usually occurs within about a week or two - but if there is little or no germination in this time, it may be useful to allow the soil to dry briefly (for a few days to a week), and once again maintaining a barely moist soil until germination begins. Several weeks following germination, you may allow the soil to go briefly dry before watering, but young seedlings should not be subjected to extend drought in their first growing season. By their first winter, the seedling mix can be allowed to remain dry for several weeks, but do not subject these seedlings to the same extended drought that I have suggested for older, more established plants. Also, do not cut off the stems of the seedling plants at the end of winter - maintain these for a least another year before you begin this annual or biennial pruning. Initially, much of the energy of the seedling will be used to grow a small tuber, so the stems may initially grow rather slowly, but in subsequent years, these will grow quite rapidly.
Euphorbia knuthii may not be the most attractive of the succulent Euphorbias, but it is perhaps the easiest of all of the tuberous rooted species to maintain in cultivation, and will, in time, produce massive, and strangely attractive tuberous roots, which is one of its most distinctive features - many growers preferring to raise the roots in repotting to display this feature. It is remarkably easy to grow this species, and a well grown plant is a sight to behold. Admittedly, this is not a plant for every grower; its spines are notorious for grappling the clothes of people who pass too closely, and like other Euphorbis species, this produces a toxic and irritating latex, making this plant unsuitable for some growers and households. Otherwise, this is an interesting plant, and is an easy plant for the beginner, and a rewarding species for the experienced grower as well.