Haworthia Form and Color Experiments
Haworthia Form and Color Experiments
Written by Miles Ridgway - Henry’s Haworthia Owner-Operator
02-30-2024
Contents:
1 - Introduction
2 - Form
3 - What Are Stress Colors
4 - What Didn’t Work
5 - What Did Work
6 - Final Remarks
Introduction
Shortly after I started collecting Haworthia plants in 2016, I quickly became obsessed with their variability in form and color. Realizing that the exact same variety has the potential to look like two completely different plants (Fig. 1–3), resulting only from their growing conditions, I began my experimentation. My goal was to reproduce, in cultivation, the dazzlingly bright reds, oranges, and yellows I was seeing in photography of wild Haworthia in their habitat. It's easy to make Haworthia change color by withholding water. The rosettes will close up, the leaves will shrivel, and they'll turn burgundy or muted red in many cases. However, that is not what I saw in wild plant photography. In nature, they are very colorful and yet appear healthy and water-filled. I tried many different things and tested many different “hypotheses” over the years. Often, what I thought would work best did not work at all. Some methods did bring out some nice colors but were not suitable for the long-term health of the plant; more on this later. The solution that I settled on is surprisingly simple. I’d like to share a little bit about what I learned through this process with other growers and collectors, or anyone who wants to see their Haworthia plants take on coloration other than just green.
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| Figure 1. Wild H. turgida var. suberecta Image credit: used with permission from haworthia-updates.haworthia.org |
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| Figure 2. Wild H. turgida var. suberecta Image credit: used with permission from haworthia-updates.haworthia.org |
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| Figure 3. Typical cultivated H. turgida var. suberecta Image credit: Our image |
Form
In order to remain compact, Haworthia plants need about 3–6 hours of direct sun, and this requirement varies by species. Other commonly cultivated succulents, such as Echeveria or Graptopetalum, also require lots of direct sun, and if they don't get enough, they etiolate fairly quickly. For Haworthia, it seems, etiolation is more of a spectrum. The amount and rate at which they stretch are highly variable and depend, of course, on light availability but also on available water, humidity, temperature, and nutrients. There are plenty of cases when they etiolate in habitat (Fig. 4), but wild plants' etiolation appears very different from the sort of rosette flattening and/or spindly deformed leaves and stems that is commonly seen in cultivated plants.
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| Figure 4. Etiolated H. transiens, DeVlugt locality Image credit: haworthia-gasteria.com |
A great majority of cultivated Haworthia are etiolated to some extent. One of the varieties that most clearly shows this is H. Cooperi cv. ‘Silver Swirl.’ Most specimens of this cultivar in circulation will be stretched to some degree (Fig. 5). However, when grown in about 6 or more hours of direct sun, they do not stretch nearly as much, and one may think that they are a different variety altogether (Fig. 6). The widespread etiolation may be due to the fact that Haworthia do not reestablish very well in direct sunlight, so transplants and props need to be kept in shade for a time. Some growers probably then do not acclimate them to brighter light, thinking that the plants like shade. For growers in colder climates, it is very difficult to provide enough artificial light indoors during the winter to prevent some etiolation.
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| Figure 5 - H. cooperi cv. “Silver Swirl” typical form in cultivation Image credit: u/mellowmark, r/succulents (reddit.com) |
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Figure 6 - H. cooperi cv. “Silver Swirl” grown in nearly full sun. Form stays compact, leaves and rosette stay smaller overall compared to specimens grown in shade Image credit: our image. |
What Are Stress Colors
Succulents develop stress colors as one means to protect their tissues and delicate organic molecules, such as chlorophyll, from the sun, among other reasons (Davies et al., 2022). The colors come from chemicals called carotenoids and anthocyanins, which are pigments responsible for bright yellows, oranges, and reds in many plants, not just succulents (Davies et al., 2022). In order to become colorful in appearance, Haworthia plants need lots of direct sun, and in order to thrive in that condition long-term, they also have to be well established so that the direct sun does not cause the leaves to shrivel. The root system must be developed enough to keep up with the plants' water demand in those intense conditions. However, direct sunlight is not enough in many cases. A great example is H. cuspidata, a hybrid between H. retusa and H. cymbiformis. They have the potential to exhibit some incredible colors (Fig. 7), but will remain green in full sun if enough nitrogen is present (Fig. 8). Since nitrogen is a major limiting factor of chlorophyll production in plants (Kalaji et al., 2017), one might conclude that when Haworthia have enough nitrogen available to replace chlorophyll as it is lost due to sun exposure, they do not need to produce as high concentrations of carotenoids, so they remain mostly green. They may only begin producing high concentrations of carotenoids and anthocyanins when nitrogen is limited. An alternative explanation may be that the bright colors are always there but are overpowered by the green pigments when Haworthia are well fertilized, similarly to our understanding of why deciduous leaves change color in the fall (U.S. Department of Agriculture, 2019).
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| Figure 7. H. cuspidata exhibiting stress coloration Image credit: Etsy.com, seller Beleafandgrow |
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| Figure 8. H. cuspidata grown in full sun, with plentiful NPK Image credit: Our image |
Worse yet, some varieties will turn a dingy brown-maroon color if they are both heavily fertilized and acclimated to full sun (Fig. 9). My, perhaps oversimplified, interpretation of the brown-maroon color is that it is the result of mixing high concentrations of green chlorophyll pigments with equally high concentrations of red, orange, or yellow carotenoid and anthocyanin pigments. Perhaps those same plants would appear fiery red if they were not fertilized so much. The question then becomes: how does one restrict nitrogen enough to cause colors to appear without adversely affecting the plants’ overall health, resilience, and disease resistance?
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| Figure 9. H. cooperi cv. 'Blue Lens' previously etiolated, adapting to direct sun, and exhibiting characteristic brown coloration. I propose that if this plant had been left in direct sun and fertilizer withheld, much of the green chlorophyll would have been lost, and, as a result, the plant's color would have changed to orange or red. Image credit: our image |
What Didn’t Work
In order for stress coloration to appear and plants to remain healthy, nitrogen must be restricted but still supplied in low concentrations, along with other macronutrients and trace elements. The availability of macronutrients (nitrogen, phosphorus, and potassium) as well as trace elements was considered with each growing medium. In natural soils, macronutrients are provided largely through the activities of soil microorganisms, involving the breakdown of organic material and the fixing of atmospheric nitrogen (FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS, n.d.). Trace elements are provided primarily through mineral weathering processes, particularly that of clay. Non-soil growing mediums, which include all gritty mixes and peat-based potting mixes, lack these natural sources of macronutrients and trace elements, so these nutrients must be supplied by the grower. If no nutrients are supplied, plants will eventually use up their stored macronutrients and trace elements, and their health and resilience will suffer. The following mediums were tested:
Pumice:
Experimentation began with plants that were already well established in a gritty mix (100% pumice). Two important facts about our own growing operation were realized: our Haworthia were overfertilized in general (Fig. 10, 11), and Haworthia can hold onto stored excess nutrients for a remarkably long time—an entire growing season at least. The control, which got no added fertilizer at all, did indeed develop some nice colors once the stored N ran out (Fig. 12), but it also stopped growing and showed signs of poor health by the end of the growing season. Other plants received various concentrations of Fox Farm's "grow-big" water-soluble fertilizer (6-4-4 NPK analysis), the lowest being one tenth label-recommended strength (about 1 mL per gallon). The plants that received liquid fertilizer, in any concentration, remained green (Fig. 13).
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| Figure 10. Overfertilized H. cooperi var. Truncata. Leaves are dark green. Image credit: our image |
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| Figure 11. Lightly fertilized H. cooperi var. truncata. Leaves are lighter green, outermost leaves showing muted stress colors Image credit: our image |
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| Figure 12. H. turgida sp. control, receiving no fertilizer in gritty mix Image credit: our image |
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| Figure 13. H. turgida sp. receiving greatly diluted liquid fertilizer in gritty mix Image credit: our image |
Bonsai Jack's succulent soil
This is a gritty mix composed of calcined clay and pine bark fines. The control growing in bonsai Jack's lasted much longer than the one growing in pumice, but eventually showed poor health and root problems similar to the pumice control. A custom-made gritty mix composed of equal parts pumice, calcined clay, akadama, zeolite, and maifan stone was also tested. The idea behind this mix was to maximize trace element availability, but it is unclear how bioavailable those trace elements are in this setting. All gritty mixes had the same issue of nitrogen balance. For optimal health, nutrients must be supplied at every deep watering for Haworthia growing in a gritty mix. One could eventually find a proper dilution of liquid fertilizer that would supply plants with just enough nutrients on a regular basis to maintain the plants' health, but it was concluded that it would be too easy to accidentally overfertilize the plants using this method. A single overfertilization could eliminate stress colors for months or an entire growing season.
Compost and peat
Eventually, store-bought potting soil would run out of stored NPK, and then stress colors would appear, but it was estimated that this could take as long as 2 or 3 years for some varieties. Since peat supplies little to no nutrients on its own, once depleted, the plants growing in potting soil would need to have nutrients added, possibly leading to overfertilization similar to the plants growing in grit. Instead, a mixture of 50%/25%/25% perlite, store-bought compost, and unfertilized peat was tested. This compost was composed of roughly equal parts composted plant material and “recycled forest products"—sawdust and ground pine bark. For this experiment, it was hypothesized that the peat-compost blend would provide a very slow-release source of NPK and trace elements and would not require water-soluble nutrients to be added by the grower. This medium yielded nice results at first (figures). However, being highly water-retentive and rich in organic material, plants had to be watered very carefully in this medium to prevent root rot. Maintaining a proper moisture level was difficult, and the medium was frequently either too wet or too dry. The mix also attracted fungal gnats, so it was not suitable for indoor growing.
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| Figure 14. H. turgida var. pallidifolia growing in perlite-peat-compost mix Image credit: our image |
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| Figure 15. H. herbacea var. lupula growing in perlite-peat-compost mix Image credit: our image |
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| Figure 16. H. cymbiformis var. obtusa growing in perlite-peat-compost mix Image credit: our image |
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| Figure 17. Young H. turgida var. pallidifolia growing in perlite-peat-compost mix Image credit: our image |
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| Figure 18. H. cooperi var. truncata growing in perlite-peat-compost mix Image credit: our image |
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| Figure 19. H. reticulata sp. growing in perlite-peat-compost mix Image credit: our image |
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| Figure 20. H. cymbiformis var. cymbiformis growing in perlite-peat-compost mix Image credit: our image |
What Did Work
It was concluded that accidental overfertilization was very easy when using any kind of water-soluble fertilizer, and commercially available potting soils or soil amendments were not suitable for growing the plants with stress colors long-term. Taking a closer look at wild plant photography revealed that the ones with the brightest coloration were all growing in what appeared to be clay or sandy clay soil, which was light in color, indicating low organic matter content. Poor-quality clay soil, unamended, was used for the next test. Haworthia turgida var. Pallidifolia was chosen. This variety can exhibit stress colors, but tends to remain green in bright conditions compared to some other varieties. The container was a porcelain pot about 12" wide, 4" deep, with drainage. The plants took about a month to become established, then slowly their stress coloration appeared (figure). By mid-summer, their color was bright, like that of similar varieties of wild Haworthia in their habitat. In addition to bright coloration, the clay-grown Haworthia showed remarkable resilience. Growing in heavy clay, root rot was a serious consideration, and at first they were watered very carefully. It was then noticed that this was not necessary. Being watered deeply seemed to pose little danger. These particular plants also show signs of water stress much later than soil- or grit-grown specimens. The clay can be allowed to stay dry for long periods of time before the plants' leaves begin to shrivel. This container is also the only one that does not need to be carefully acclimated to direct sunlight in the spring. The clay-grown Haworthia can be brought from artificial light to direct sun and adapt immediately, but potting mix or grit-grown plants would certainly have suffered sun scalding. The leaves remain water-filled, and the color changes from a pale lime-green to shades of yellow and eventually orange within about a month after being moved outdoors to full sun. The trade-off for bright stress coloration and resilience appears to be that the plants grown in clay without fertilizer grow incredibly slowly. After 3 years, the well-established H. turgida var. Pallidifolia plants only have a few tiny offsets. Potting mix or grit-grown specimens of the same variety, given fertilizer, would have grown into massive clumps in the same timeframe. It is unclear whether Haworthia would grow well in smaller containers being planted in clay or whether the larger size of the container used in the experiment was a factor.
Final Remarks
What I originally wanted was to find a way to bring out the stress colors in my entire collection and allow them to continue growing at a moderate rate, but I do not think that is possible. Some varieties show stress coloration much more readily than others, but for many varieties, they will remain green, even in full sun, if enough nitrogen is present. For those varieties, restricting nitrogen enough to allow their stress coloration to come out also greatly slows their growth. As a grower, I may still choose to lightly fertilize my plants, making it easier for buyers to bring out the plants' stress colors if they so desire. After years of “over-teching it,” I found that the best, and arguably only, way to grow healthy yet beautifully stress-colored Haworthia long-term is to grow them in nutrient-poor clay soil from outside. Potting mix would technically work if no fertilizer was added, but plants would need to be repotted with new potting mix every 2–3 years to keep them healthy. Even if I did find the proper dilution of water-soluble fertilizer, I am led to believe that the nutrient-stressed plants would still not grow as resiliently in potting mix or grit as they do in clay, likely due to the balance of macronutrients and trace elements that natural clay soil provides. Instead of trying to stress my entire collection, I must be content knowing that in order for them to grow quickly, they need nitrogen, and therefore most will remain green. What I may do instead is create more “micro habitats,” similarly to the process of Bonsai, using my favorite Haworthia species. Stress colors can most likely be achieved in virtually any growing medium, for a time. Even commercial potting mix, which is very nutrient-rich, would eventually run out of stored NPK, and then stress colors would appear, but I estimate it could take as long as 2 or 3 years after the plant becomes established. Then those plants growing in depleted potting mix or grit may become increasingly vulnerable to pests or disease or root problems, as mine did throughout the experiment. Future experiments may involve finding a dilution of water-soluble fertilizer that does not quickly eliminate stress coloration when applied and testing whether container size or shape affects plant health when growing in clay.
Works Cited
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