Introduction
Haworthia is a genus of succulents from South Africa related to Aloe and Gasteria. Within the genus is a group of closely related species known as the retuse haworthia. During the last 20 years a devout following for retuse haworthia has arisen as more of these diverse jewel-like plants have become available to collectors.
Retuse haworthia have flattened leaves with leaf windows. Many have intricately decorated leaves and windows that, once appreciated, make the plants irresistible. Leaves can be shiny (figure 1), spiny (figure 3), spotted (figures 2 and 4), translucent (figure 1), and can take on a range of nearly indescribable colors ranging from the ghostly bluish-gray seen in some H. emelyae ‘major’ plants to the deep red and brown hues of H. mirabilis ‘badia’ to the oranges seen in some H. emelyae ‘picta’ and H. emelyae ‘sandkraalensis’ to the bubble gum pinks seen in H. mirabilis ‘splendens’.
1
Haworthia mirabilis ‘badia’ can have shiny translucent windows with deep red coloration. This dramatic individual was originally obtained from Steven Hammer.
2
H. mirabilis ‘splendens’, from a locality west of Albertinia, can be jewel like with shiny leaves and dense pink spotting.
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H. emelyae ‘sandkraalensis’. This individual was originally obtained from Mary Parisi and is from a population where some individuals have dramatic orange coloration. This plant is a great example of window tissue bleeding into the leaf throat.
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H. mirabilis ‘enigma’, a choice variety. This individual has layers of red and green pigmentation and spotting overlaying dark blackish leaves.
I have been hybridizing haworthia for roughly 15 years. Many interesting hybrids have come out of Japan over the years, and these were an early inspiration. I have also been stimulated and influenced by other hybridizers, including Mary Parisi, Bob Kent, and, especially, Steven Hammer, who in addition to being an inspiration has provided me with a plethora of good plants to use as breeding stock. I have also incorporated their hybrids into many of my own. In 1998, this journal featured two special issues devoted to the genus Haworthia (July–August and September–October). They contain some particularly interesting articles and I recommend them to anyone interested in the genus.
Hybridization has a somewhat bad reputation among some succulent enthusiasts as promoting generic characteristics and masking differences between species and localities. While it is true that in some instances hybridization can produce dull plants, my experience with haworthia hybridization suggests that carefully (and occasionally not so carefully) planned crosses of haworthias can result in striking new variations. For me, hybridizing haworthias has been a satisfying intersection of the botanical and the artistic.
Selection of good parent plants for hybridization is important for obtaining optimal results. The selection of which species to use is not as crucial as the selection of individual plants which have interesting traits to contribute to a hybrid. Using high caliber parent plants such as the ones in figures 1–4 can increase the chances of producing desirable results.
The traits I have focused on are leaf shape, color and texture, and window morphology. Through hybridization these can be combined, enhanced and superimposed to create something unique and beautiful. For example the pink pigmentation of H. mirabilis ‘splendens’ in figure 2 has been enhanced and combined with unique leaf morphology in my “Pink Floyd” hybrid (figure 5).
5
Haworthia “Pink Floyd”, a hybrid that enhances and transposes the pink coloration of H. mirabilis ‘splendens’ in a plant with a different leaf shape.
Types of hybrids.
I encourage readers interested in hybridization to read up on Mendelian genetics. A basic understanding of the concepts can help one understand how genes and traits behave in hybrids. The following definitions will help introduce the pertinent concepts.
F1 hybrid: Offspring resulting from crossing two different parents. Depending on the cross, an F1 population is usually uniform. F1 hybrids contain one copy of every gene from each parent. Because of this, a desirable trait in one of the parents caused by a particular gene can be masked by the gene from the other parent. As a consequence of this kind of genetic interaction, F1 hybrids sometimes lack interesting traits seen in the parents. F1 hybrids are known to display hybrid vigor (improved fitness seen in hybrid offspring) and, in my experience, this is often the case in haworthia F1 hybrids.
F2 hybrid: Offspring from self pollination of F1 hybrids, or from crossing two F1 plants of the same cross. F2 siblings are highly variable in appearance. Due to independent assortment of genes (Mendel's second law), parental traits can become unmasked in F2 populations. F2 hybrids are thus often more attractive than F1 hybrids, as traits from the grandparents are expressed, but their beauty comes at a price. Hybrid vigor is lost in F2 hybrids and, in my experience, F2 haworthia hybrids can be slower growing, smaller in size and sensitive to root loss.
Backcross: Crossing an F1 hybrid to one of its parents. This can be used to create hybrids with more characteristics from one parent or the other.
Complex hybrids: By carefully crossing different hybrids and species together the results can often be dramatic. The best results are usually obtained when both sets of parents in the cross have similar traits, such as coloration, that one wishes to enhance.
Haworthia leaf traits
Anatomy of a haworthia leaf.
One of the ways retuse haworthia leaves differ from the leaves of other haworthia is that they tend to have more distinct facets. I will define three different domains in the typical retuse haworthia leaf: the leaf windows, the leaf throat and the leaf backs (figure 6). Most retuse haworthia have triangular-tipped leaves. Leaf tips may be pointed as is the case with H. mirabilis ‘badia’, or they may be blunt as is the case with H. bayeri. The leaf structure may be raised somewhat, producing a chunky appearance, as with H. mirabilis ‘splendens’, or may be flattened, as is the case with H. emelyae.
Leaf windows.
One of the distinctive features in retuse haworthia is the presence of translucent windows on the leaves. Windowed leaves have evolved independently in many distantly related plant groups, including mesembs and peperomias. Their function remains a mystery, but they are likely an adaptation to arid environments, and may help camouflage the plants. I believe that windows evolved as a way for the plants to dissipate heat when it is too hot, and to capture heat from the sun when it is cold. I have noticed that on sunny cold winter days, windowed haworthias seem to radiate warmth if you hold your hand near the windows. Whatever their function may be, leaf windows are a captivating feature of these plants.
Haworthia windows are a mix of two different tissues, the translucent tissue (similar in appearance under the microscope to the storage tissue inside the leaf), and lines or veins (similar in appearance under the microscope to the tissue in the leaf throat and leaf backs). The lines innervate the windows from the border between the window and the leaf throat. Bubbles of window-like tissue sometimes speckle the leaf throat. This trait expresses itself in species like H. mirabilis ‘badia’, H. emelyae and H. emelyae ‘major’. H. emelyae ‘sandkraalensis’ in figure 3 is a good example of window tissue bleeding into the leaf throat. Speckles of window-like tissue can also sometimes also appear on the leaf backs. This can be seen in the plant in figure 6.
Some retuse haworthia diverge from this basic leaf shape and window configuration. H. truncata ‘maughanii’ and H. truncata are the most obvious examples of this phenomenon. Instead of triangular tips and windows, H. truncata maughanii has rounded leaf tips and windows, while in H. truncata the leaf tips and windows are oval (figure 7). In H. truncata the phyllotaxy (arrangement of the leaves on the stem) is different from other Haworthia species, forking in a distichous (fanlike) pattern, similar to many gasteria species, rather than forming a rosette
One of my hybridization goals has been to create plants with unusual leaf shapes and window morphologies. Crossing H. truncata with other retuse haworthia can result in such hybrids. Figure 8 shows an F1 hybrid between H. truncata and H. mirabilis ‘badia’. This hybrid was interesting on its own, but I wanted to introduce more pigmentation while retaining the unusual leaf morphology. To accomplish this goal, I crossed this F1 hybrid to H. mirabilis ‘splendens’. The progeny from this cross, named “Obama” in honor of our president, expressed a wide range of different leaf shapes and colors (Figures 9a–d).
8
A hybrid between a brown colored H. truncata and H. mirabilis ‘badia’. In this hybrid, the rosette leaf arrangement of H. mirabilis ‘badia’ is dominant. Leaf shape is intermediate between the two parents.
9a–f
H. “Obama” (mirabilis ‘splendens’ ×(H. truncata ×H. mirabilis ‘badia’)) siblings. a In this plant the H. truncata grandparent contributes wide leaf morphology and H. mirabilis ‘splendens’ contributes white coloration. b This plant has brick red coloration and thick window lines. c This plant has a leaf shape and window morphology similar to the H. mirabilis ‘badia’ grandparent but with a more unusual look. d In this plant spotting of H. mirabilis ‘splendens’ expresses itself as an opaque white overlay in the windows and leaf throat. e Wide shiny chocolate colored leaves. f Wide leaves with pink bumps.
In addition to hybridization with H. truncata, crossing any two haworthia species with slightly different leaf and window configurations can occasionally result in hybrids with unexpected leaf morphologies, including a more diffuse window-leaf throat boundary or thick lines on the windows (Figures 9b, 10–11). Sometimes the windows are pushed to the edges of the leaf tips. This effect, not normally seen in un-hybridized plants, can be observed in the plant in figure 10.
Leaf coloration.
The coloration in some haworthia varieties can be beautifully complex. I have observed several layers of pigmentation including overall leaf coloration, leaf overlay coloration, leaf window spotting, and leaf throat coloration. The H. mirabilis ‘enigma’ in figure 4 provides a good example. The epidermal coloration is a dark blackish green, the overlay color is red, the windows have spotting, and the leaf throat has an almost fluorescent green hue. Coloration can vary greatly within species and between plants from the same population. For this reason, I have found that it is important to use individuals with the most intense traits to achieve the best results.
The overall coloration, and especially the overlay tint, are influenced not only by genetics, but are also strongly influenced by horticulture, where light seems to play the biggest role. In the absence of adequate light (1 to 2 hours of direct morning or late afternoon sun is sufficient), most haworthias are a disappointing flat green; however, with adequate light, their colors develop, sometimes dramatically, and a once unimpressive and dull plant is transformed into a precious jewel.
Some retuse haworthia have spots on the leaf windows. The spots are white but may appear red, pink, or orange depending on the overlay coloration (N.B. when grown in the shade, the spots are uniformly, and blandly, white). The spotting morphology differs depending on the variety. Spots may be raised and bumpy or smooth. Spot size varies from plant to plant. The largest spots seem to be seen in H. mirabilis ‘splendens’, while in different H. emelyae varieties a range of spot sizes can be observed. Some haworthia, like H. emelyae ‘major’, have diffuse fine leaf spotting that manifest as a white overlay in the windows. In certain crosses, the bumpy spotting of H. mirabilis ‘splendens’ is transformed into more of an overlay spotting like that seen in H. emelyae ‘major’ (figure 9d). From my observations, the inheritance of spotting is genetically complex. The trait appears to be controlled by the same genetic determinants in different species, since crossing different spotted species, for example H. mirabilis ‘splendens’ and H. emelyae ‘picta’, will produce mostly spotted progeny with an occasional unspotted plant. I have found that, in certain crosses, these traits disappear and cannot be recovered in subsequent generations, even by backcrossing. By selecting and crossing the most highly pigmented plants, I have been able to intensify the pigmentation and spotting traits, sometimes in plants with unique leaf shapes. Figures 12–15 are plants bred to have intense leaf coloration.
12
H. “scarlet begonias” (H. mirabilis ‘splendens’×H. emelyae ‘picta’). The leaf shape is similar to the H. emelyae ‘picta’ parent. The leaf spotting and coloration is more intense than either parent with pigmentation in contrasting shades of red. This hybrid is a sibling of the plant in figure 11 and was used as a parent to impart red coloration to the hybrids in figures 13, 15 and 20.
Leaf texture
Window leaf texture.
Leaf windows in retuse haworthia may be smooth or rough. Many plants that appear to have smooth leaf windows (for example H. mirabilis ‘badia’, H. bayeri and H. mirabilis ‘splendens’), may in fact have little bumps upon closer examination. In some varieties, most notably H. emelyae ‘major’ but also some H. pygmeas and H. emelyae ‘sandkraalensis’, the leaf window cells have spines (see figure 16a–b). This spination feels rough to the touch and can give the plants a frosted appearance.
Spination seems to be a semi-dominant trait. When spiny plants are crossed to non-spiny plants, the hybrids tend to have spines that are much shorter than the spiny parent. In subsequent generations the long spines are recovered if these plants are crossed together.
In my breeding I have worked on increasing spination, transposing spination into a different leaf context and combining spination with increased pigmentation. Examples of my spiny hybrids can be seen in figures 17–20.
17
H. “pricklypete”. This is a complex cross involving a blue-grey H. emelyae ‘major’ and two hybrids developed by other people, H. “frosty” and H. “Bev's wonder”. The spination of H. emelyae ‘major’ is accentuated with white pigmentation and a unique leaf shape.
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A hybrid of H. “korizato” (a Japanese hybrid and early inspiration to me) crossed to H. emelyae ‘sandkraalensis’ (figure 3). The spines have an unexpected red coloration.
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F2 hybrid between H. emelyae ‘major’ and H. bayeri; in this hybrid, the spines are large and accentuated against a dark background, producing a visual effect similar to ice crystals.
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A complex hybrid that brings together the spination of H. emelyae ‘major’ and the coloration of H. mirabilis ‘splendens’ and H. emelyae ‘picta’ in a plant with wide leaves and a tight rosette.
For information about the actual mechanics of pollination I refer readers to the following section on my website: http://www.haworthiahybrids.com/pollination.html
