Liriope’s Muse: Tree Care Tips from a Master Arborist
TRUSTED TREE CARE SERVICES SINCE 1970
Liriope's Muse - Mistletoe: The Romanticized Parasite
Mistletoe has been present in Texas for decades, but in the recent months our arborists have noticed a vast uprise in mistletoe cases across the greater Houston area. While it has always occurred locally, we have not previously seen it at the frequency, or severity, we’re encountering now.
The Mistletoe is parasitic to over 1,300 tree species worldwide and more than 30 in Texas alone. In our service area, however, we are finding it on various oaks, elms, sugarberries(hackberry), and several types of pines.
*Above are photos taken by our arborists in the field. The left-most photo is a close-up photo that our arborist took of mistletoe in the crook of a tree. If you look closely, you can see what the mistletoe's attachment to the tree looks like. The two right most photos were taken of a deciduous oak tree with a heavy infestation of mistletoe*
American mistletoe (Phoradendron spp.) is an evergreen parasitic plant, that also has the unique ability to photosynthesize which categorizes it as a hemiparasite; meaning it obtains part of its nutrients and energy from a host and the other part from photosynthesis.
In East Texas, mistletoe is dioecious, with male and female flowers borne on separate plants. The flowers are small and creamy-white, and only the female flowers produce seed. Those seeds are white and surrounded by a sticky, gelatinous pulp, which helps them adhere to bark after they’re dispersed. Via hitchhiking on (or in) an animal, transported by humans when the plant is removed from the tree, or by explosive discharge. (Some species have the unique ability to shoot seeds from their berries up to 50ft away and at 60mph)
Trees aren’t completely defenseless to mistletoe. In fact, in order for the tree to become under attack by mistletoe, it must first be weakened and stressed by some other factor (such as construction damage or drought). The seed must also land in the right location; if it lands on thick mature bark, the haustoria (the plant's root-like structure) will not be able to penetrate the thick, hard surface. In order for mistletoe to successfully inhabit a tree, it must land on newer growth where the bark is thin and soft enough for the haustoria to penetrate and enter the bark through a lenticel or bud and make its way into the trees vascular tissue.
Once a mistletoe seed lands on a suitable host, it enters a gradual and slow germination phase that takes 2-3 years. In this stage, mistletoe relies heavily on its own photosynthetic capacity to produce its energy while it works to gain purchase in the host. First-year ariel growth is very minimal (often >0.5 inches), as the plant prioritizes building a functional haustorium, its attachment and uptake structure, before it invests in visible canopy growth. Once it reaches the host’s conductive tissue, it begins to grow and mature much quicker (some species can reach up to a 3 foot spread in just 6-8 years).
The Romanticized Vampires:
Mistletoe is thought to have been first romanticized by Celtic Druids in the first century A.D., in part because it remained green and thriving through winter conditions. When trees had dropped their leaves and the landscape appeared dormant, mistletoe could still be seen flourishing in the canopy. This persistence led the Druids to regard it as a sacred symbol of vitality, and they reportedly used it in remedies for both people and animals, believing it could help restore fertility.
Mistletoe’s associations with fertility and vitality continued through the Middle Ages, and by the 18th century it had become widely incorporated into Christmas Celebrations. Just how it made the jump from sacred herb to holiday decoration remains up for debate, but the kissing tradition appears to have first caught on among servants in England before spreading to the middle classes.
As part of the early custom, men were allowed to steal a kiss from any woman caught standing under the mistletoe, and refusing was viewed as bad luck for her future marriage. Another common tradition involved plucking off a white berry for every kiss taken under the mistletoe, once the berries were gone the smooching ended. In this custom, each berry on a sprig of mistletoe had come to represent a kiss that a man was allowed to bestow upon a young woman standing underneath the plant, and “when the berries are all plucked the privilege ceases.”
Now that we have an understanding of the most well-known love themes associated with mistletoe, lets address why they are ‘vampires’.
Mistletoe is a parasite that sucks the life from branches of its host, by inserting its fang-like haustoria into an open bud or lenticel on the bark and entering the trees vascular system where it draws out water, minerals, and the trees essential sugar reserves. This parasitic siphoning may not solely kill your tree, but it can lead to a ‘vascular tug-of-war’ that gradually weakens and stresses the host leaving it susceptible to host more serious pests and pathogens that WILL kill your tree. Such as bark borers or a vascular disrupting fungal pathogen like phytophthora.
Management:
Mistletoe can live in a host tree for over 100 years and has few natural enemies, so effective control is very difficult.
Mistletoe plants can mature as soon as 3 years, and mechanically removing the aerial portion of the plant before it matures and produces seeds can benefit some as you’ll be less likely to spread its seed if it has not yet produced any. That said, mechanical removal can be attempted at any point in the plant’s life cycle, but it should be noted that the mistletoe will continue to regrow from its established haustorium. (Removing part of the wood where the mistletoe attaches to its host should be avoided as it causes more damage than the mistletoe itself.) Mechanical removal is only suggested if it can be done safely and economically, like if it is a serious infestation, constantly removing the hard-to-reach mistletoe from your tree may prove to be more difficult than it is worth it.
The one thing that can be done in an attempt to prevent regrowth of the mistletoe after its removal, which harms the tree the least, is to wrap the removal site on the branch in a thick black plastic for at least a year. In theory, this would block the sunlight and oxygen and the plant would eventually die from this but there are a few issues with this method. Firstly, this can be damaging to the tree if wrapped too tightly and not closely monitored; it also blocks light and oxygen from that area of the tree too. Second, plastic degrades so it’ll need to be regularly changed and maintenanced so as to ensure zero light at any point penetrates or else it’ll have been ineffective. So not only is this process stressful to the tree, expensive, and labor-intensive, but it is also very unsightly and, in the end, may not even have been effective!
Some think that herbicides may be effective for mistletoe control, but they are very harmful, more so than the parasite itself. Since the plant is intertwined with the trees vascular system, when it is sprayed with an herbicide, it’ll back feed into the tree and the tree will also be poisoned. Herbicides should NEVER be used in the control of Mistletoe!
Small, infested limbs can be removed by pruning and have been shown to be successful at the removal of the parasites from the tree without further spread. But large limbs should never be removed from the tree as it can cause serious damage
So, the next time you think about sneaking a kiss with your lover under a mistletoe, think about how it has spent its life, draining the life from a tree!
Fun Facts I Found During My Research:
- Ancient Anglo-Saxons noticed that mistletoe often grows where birds leave droppings, which is how mistletoe got its name: In Anglo-Saxon, “mistel” means “dung” and “tan” means “twig,” hence, “dung-on-a-twig.”
- As they mature, mistletoes grow into thick, often rounded masses of tangled branches and stems, which can often cause arborists to mistake them for “witches brooming”. These tangled masses can reach 5-feet wide and weigh 50 pounds.
- Three kinds of U.S. butterflies depend on mistletoe for survival: the great purple hairstreak, the thicket hairstreak, and the Johnson’s hairstreak. These butterflies lay eggs on mistletoe, and their young eat the leaves. The adults of all three species feed on mistletoe nectar, as do some species of native bees.
- The mistletoe’s white berries are toxic to humans but are a valuable source of food to other mammals species like: deer, elk, squirrels, chipmunks, and porcupines.
- Extracts from mistletoe are newly being used in Europe to combat colon cancer, the second greatest cause of cancer death in Europe and the Americas. This treatment research is showing signs of being more effective against cancer and less toxic to humans than standard chemotherapy.
Resources:
https://blog.nwf.org/2012/12/12-things-to-know-about-mistletoe/
https://www.history.com/articles/why-do-we-kiss-under-the-mistletoe
https://tfsweb.tamu.edu/trees/tree-health/whats-on-my-tree/mistletoe/
Liriope’s Muse - Expert Tree Care Tips
