An Ironclad Plant for Winter Enjoyment
The cold weather of January provides an excellent opportunity, not to mention a great excuse, to remain indoors and focus on houseplants. Although the warm home environment is great for the gardener, the low humidity and the reduced light levels make it a challenge for plants – especially if they recently enjoyed the summer in the humid out-of-doors. Fortunately, for avid gardeners and novices alike, there remain a few plants that thrive in these harsh environments and actually grow best under neglect. One such plant that I have admired since my youth is the Jade Plant, Crassula ovata.
Jade Plant is a member of the Crassulaceae or Stonecrop Family that features among its clan of 35 or so genre a number of ornamentals including Sedum, Echeveria and Aeonium. Crassula contains around 200 species and was named by Carl Linnaeus (1707-1778) in 1754. The name stems from the Latin Crassus, meaning thick and refers to the thickened foliage and stems that are characteristic of many of the species. Most species of Crassula, including Jade Plant, are succulents, inferring that they can store water in their fattened leaves, stems and occasionally even in the roots. Crassula as a genus has a wide spread distribution, with species native to the more arid regions of North America, Europe, Africa, New Zealand and Australia. However, the center of the world’s population lies in South Africa north into Mozambique, which is home to Crassula ovata. The species epithet of ovata is from the Latin meaning egg-shaped and aptly describes the shape of the foliage. Jade Plant was first brought to England in 1759 and in 1768 was named Cotyledon ovata by Philip Miller (1691-1771). Miller was the head gardener at the Chelsea Physic Garden from 1722-1771 and taught many budding gardeners, including William Forsyth, for whom Forsythia is named! It was not until 1916 that George Claridge Druce (1850-1932), an English Botanist and mayor of Oxford properly reclassified the genus as Crassula. The oblong foliage is roughly 1” long by ½” wide and its glossy, jade green color was undoubtedly the basis for its common name.
Jade Plant is certainly not rare! Arguably, many gardeners consider this plant to be a dull, if not an outright boring windowsill plant! I suspect the problem lies in our impatience, as we typically abandon or kill the plant through the kindness of over watering before it reaches the elegant stages of its life. Plants need to be grown for 15-20 years before they start to assume the majestic appearance of a well weathered tree with a strong stout trunk (pictured at right). The key is not to over water and only up-pot to a marginally larger container, since excess potting soil can easily become waterlogged. Water very conservatively during the winter, when growth is at a minimumm due to the lower light levels. If the plant receives ample direct sunlight throughout the year, you will be rewarded with clusters of pink-blushed white flowers at the tip of the stems throughout December and January (pictured at right)! With greater exposure to sunlight, the leaves also develop very attractive colorations of red and yellow, which becomes most pronounced if the temperatures do not exceed 60 degrees (pictured below). A good excuse to keep a room cooler throughout the winter! The production of these pigments is actually a defense mechanism against the harmful effects of UV radiation, reminiscent of people developing suntans in the summer. The selection that is called ‘Variegata’ or ‘Lemon and Lime’ has very attractive light yellow to white streaks of variegation running from the base of the leaf to the tip. Often, these leaves will take on purple shades if it receives more sun. An even more unusual leaf shape is found on the selection called ‘Gollum’, which bears no resemblance to the character found in J. R. R. Tolkien's The Hobbit or Lord of the Rings. In this case, the leaf is rolled into a cylinder with a depression at one end that appears much like a suction cup at the one end! With ample sunlight, the tips of the leaves become very attractively brushed with red (pictured at right)!
Another Crassula that I have long appreciated for its unique appearance and ease of culture is Crassula perforata, commonly called String of Buttons. The species epithet is from the Latin Perforo, meaning to pierce or make a hole, which describes how the stem appears to pierce or pass through the button-like leaves! In reality, the ½” long, heart-shaped leaves are arranged oppositely on the stems and are fused at the base, giving the illusion the stem passes through the leaf. This species is also native to South Africa, and was appropriately named in 1778 by the Swedish naturalist Carl Peter Thunberg (1743-1828). Unlike Jade Plant, the wiry stems initially grow upwards to 12-18” tall before collapsing to the ground. The tips of the stems once again begin their upward ascent and the ‘dance’ is repeated. ‘Variegata’ is an attractive form featuring broad, off-white margins along the foliage (pictured at left). The variegation is most defined when the plant receives at least 4 hours of direct sunlight, otherwise the margins revert to a light green.
Crassula tetragona is once again a native of South Africa and it appears totally different than its previous two cousins. The upswept, needle like foliage grows to 1” long and is arranged oppositely along the stems. What makes it unique is how the foliage immediately above or below a given pair of leaves is rotated 90° to this set of leaves, giving it the common name of Four Angled Crassula (pictured at right). The leaf arrangement was also the inspiration for Linnaeus who named and described the plant in 1753. The species name is from the Greek Tetragonos, meaning four equal angles. The foliage also bears a resemblance to that of a pine tree, giving it another common name of Miniature Pine Tree! In fact, this Crassula is a popular candidate in bonsai for crafting windswept pine trees. The plants will gradually grow to 3 feet tall with the bark assuming an attractive cinnamon brown color. True to form, plants resent overwatering and will provide clusters of the off-white flowers at the tips of the stems in January when given ample sunlight.
Although all three of these Crassula species look totally different, they all employ an unusual and truly ingenious technique for conducting photosynthesis – they collect Carbon Dioxide (CO2) at night, not during the day! Initially, this may not sound like a brilliant adaptation, but for a plant growing in an arid location, it provides a great advantage! Most plants have openings on the bottom of their leaves called stomata; they open during the day, allowing CO2 to enter and the byproduct of Oxygen (O2) to escape while at night they close since the process of photosynthesis only occurs in the presence of light. This process always comes at a cost to the plant, since the warmth of the sunlight striking the leaf turns the water inside the leaf to vapor, which in turn is lost through the stomata openings. This is an acceptable process if water is readily available, but far from desirable if water is in short supply! Providing a solution to this dilemma, plants in the genus Crassula and numerous other genera flourishing in these arid environs developed a system called Crassulacean Acid Metabolism or CAM for short. This is a technical mouthful, but it is obvious that the genus Crassula is at the center of this metabolic anomaly! For these plants, the stomata open at night, when the relative humidity is higher and, in the absence of the sun’s irradiation, the leaf is far cooler, reducing water loss. The O2 is obviously released, but the CO2 is temporarily held in the chemical forms of Malic and Isocitric Acids, allowing greater concentrations to accumulate than if it were simply to remain as a gas within the leaf. Come sunrise, the stomata close, the acids break down and the CO2 is released within the leaf, allowing photosynthesis to occur. Simply brilliant! Obviously, this is less efficient or beneficial to plant growth, since only a portion of the CO2 can be stored chemically as compared to plants with more ‘traditional’ photosynthetic mechanisms. In part, this also explains why plants with CAM processes have a more stunted growth rate. However, it allows survival! During cases of extreme drought, these plants will endure CAM-idling, in which the stomata do not even open at night and the plant uses the CO2 released from cell respiration for photosynthesis and the O2 from photosynthesis for respiration. This process does not allow for the plant to experience growth, nor even sustain itself for very long, but it does allow it to endure short periods of extreme drought.
Plants with iron clad constitutions that not only survive, but thrive under the challenging home environment should not be looked upon as boring, but given a place of respect. The genus Crassula certainly lives up to this challenge, with the Jade Plant serving as the family patriarch as it grows ever more elegant with age and offers the opportunity for winter bloom when the conditions are proper (pictured below). It should also garner great respect for the fascinating photosynthetic pathway by which it creates sugars. With few insects other than Mealy Bug to serve as a potential problem, this is an ideal group of plants to enjoy during the cold days of January, and for the remaining eleven months as well!
Director, Rutgers Gardens