Beautiful Native Plants

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Thursday, June 30, 2016

The magic of the mistletoes

Globes of mistletoe become more evident in the
winter when the host tree loses its leaves.
This is one in my yard that is using
 a black gum (
Nyssa sylvatica) as its host.
Mistletoe played an important role during ancient times in the Old World when people brought evergreens inside so the gods of spring would have safe haven for the winter; and they celebrated the beginning of the return of the sun with various festivals. The European mistletoe (Viscum album) played and important role in these pagan rituals.

You can understand how the ancients must have been mystified by mistletoe. It stays green while the host tree apparently dies over the winter. It never touches the ground, so it must be purer somehow. Some thought that it must be a plant graft from the gods.  See my article, Myths of mistletoe to learn about some of these legends.

It is the synthesis of various legends that led to the tradition of kissing under the mistletoe for good luck, fertility, and/or long marriage. While mistletoe has been used for medicinal purposes over the centuries, both the leaves and berries are toxic to both humans and pets.  Unlike poinsettia, that New World Christmas plant, which despite its reputation, is not poisonous. See my article, Poinsettias are NOT poisonous.

A hemi-parasite

American or eastern mistletoe (Phoradendron leucarpum) is a shrublet native to the whole southeastern section of the US from New Jersey to Florida and is most similar to the European species.  Like all mistletoes, it is a partial parasite (hemi-parasitical, to use the botanical term), because it must grow from the branches of a host tree. It has special root-like system called haustoria that invade the host tree to extract moisture and whatever nutrients are in the tree’s system. Because this mistletoe has green leaves and produces its own sugar through photosynthesis, it is not a complete parasite. Some other species of mistletoe do not photosynthesize to the same extent and may cause more damage to their host trees.

There are several species native to the U.S.  There are two species in Florida: the American mistletoe (P. leucarpum) found in most of the state growing mostly on oaks, and the mahogany mistletoe (P. rubrum), which only grows in the southernmost counties.  The western mistletoes generally have fewer leaves and photosynthesize through their stems. The red berries and stems on the California mistletoe (P. californicum) are striking in the southwestern desserts and you would not guess that it’s related except for its hemi-parasitical habit.

California or mesquite mistletoe (P. californicum) is striking with its red stems and red fruit. It grows in the SW deserts on mesquite or acacia shrubs.

You can estimate the age of a mistletoe plant by the number of branch divisions.  Count the number of forks in a single branch to its end and then add two to that number for the age.  It takes a year for the haustoria to invade the host and develop its network of cells to accept the fluids and the second year it sprouts without any branches. When you see the size of some of the mistletoe globes, they are probably more than 50 years old.

The point of attachment is swollen where the host tree has tried to seal off the wound created by the haustoria. This mistletoe is using a water oak (Quercus nigra) as its host.  There is damage to the host, because this node is a weak spot in the wood, but the question is, does the host also receive sugars and other benefits from its guests.

If you think about the physics of mistletoe, it has to keep the water and nutrients flowing through its host’s xylem cells even when the host is otherwise dormant.  Transpiration, the flow of water into the roots, up the xylem tubes, into the leaves where most of it evaporates into the air through the pores (stomata), is a passive process driven by the big suck of the evaporation from the leaves.  Because of the complexity of the haustoria and all of their plumbing cells to keep transpiration going throughout the winter, it would follow that the mistletoe might also store some of its sugars to benefit the host plant.  So maybe there is some symbiosis going on here–what an interesting research project that would be.  For more information on transpiration see my post, Water science and gardening.

Mistletoe needs birds 

For decades nursery people have tried to cultivate mistletoe to simplify its harvest for the Christmas trade, without much success.  The seeds are very sticky (hence the family name Viscaceae for viscous) and as the birds eat the fruit, the seeds stick to their beaks, which they wipe onto the tree bark where it sticks.  Alternatively, the seeds are eaten and are then deposited on a branch by birds with a dollop of fertilizer.  Anyway, at this point mistletoe is not in cultivation so the folks who harvest it for us have to work hard to retrieve it.  They shoot it down with a gun, climb the trees to chop it down manually, or use cherry pickers to get high enough to harvest it.  This may explain the high prices you see.

And birds need mistletoe

Foresters used to consider mistletoes pest plants that ruin trees, but it's been shown that the mistletoes play and important role in forest ecosystems and trees actually do better where mistletoes are present. Birds, especially migratory birds, depend on mistletoe berries, which are available in an off season from most other plants. Insects and other soil critters depend upon the nutrients found in the fleshy leaves that drop on on a year-round basis. When researchers considered all the studies, they determined that mistletoes are a keystone species in their ecosystems.

There are several globes of mistletoe on our property growing on oaks and black gums and I love it in the deep winter (such as it is here in north Florida) when flocks of cedar waxwings come through and work over all the mistletoe plants.  What better holiday décor could you want?

So mistletoe is not just for Christmas

When you do celebrate the winter solstice and all its associated holidays, it gives us hope when we think about the magical and mythical mistletoe and how it stays alive even while its host appears to be dead.

© Ginny Stibolt