Tightly curled in protection, the leaves of Rhodendron exhibit the ‘droop and curl’ of thermonasty. This allows the shrub to survive winter winds and light. Hawk Mountain Sanctuary, PA

Whether you watch TV, listen to radio or just venture into any retail establishment you are well aware of the many elixirs, potions and formulas available to cure all sorts of ills both real and perceived. The same is true in the plant world. Any garden center you visit will have shelf upon shelf of chemicals, organics, salves, sprays, drenches, repellents, amendments and the like.

One that I have never understood is the use of Wilt-Pruf and other anti-dessicants on plants in the landscape. Sure, I sold it to folks as a young garden center employee without any knowledge of the way plants work, but were I in the same position today I would tell the friendly garden center shopper to save their money. Mother Nature’s got this…

Most requests came as the cold settled in for the winter. As the days shortened people began noticing their broadleaf evergreens looking less than perky. We all know how dry it gets in the winter, and for the plants, moisture that is normally available to them is now in a frozen state, not available to be drawn up through the roots. And much of the falling moisture is is frozen, slow to melt and percolate down to thirsty roots. Winter winds don’t help things out much.

Rhododendrons on an exposed ledge at Hawk Mountain Sanctuary in PA. Curled leaves protect these plants from sunburn and moisture loss even in this exposed location.

Trees like Hollies and shrubs like Rhododendrons and Mountain Laurels (Kalmia) are known as Broadleaf Evergreens, this is in comparison with needled evergreens like Pines, Spruces, Yew and Juniper. Deciduous trees (which lose their leaves all together in winter, primarily to reduce moisture loss) and needled evergreens really have the winter advantage. Needled evergreens’ skinny leaves (yes, needles are modified leaves) are coated with a waxy layer. This combination of reduced surface area and natural waxy protective coating keeps winter gusts from sapping the little moisture they have right from the leaves. Broad Leaf evergreens have a little bit of a tougher time in winter. They do have the waxy coating (called a cuticle) but their surface area is far greater. This anticipated desiccation is the “ill” my garden center visitors are trying to cure with a potion in a bright green and white bottle.

Many people take pity on these poor rhododendrons, thinking they are doing good by applying a salve to coat the leaves. But as is the case with many ‘ills’ both in the human and horticultural world, nature has a way of managing what’s going on.

To understand what Mother Nature is doing to keep these lovely shrubs alive in our weather extremes, it is good to take a look at where these beauties live.  Rhododendrons and their kin have been on this planet for nearly 50 million years (a lot longer, by the way, than the patent for any horticultural anti-dessicant) and have managed to survive the extremes Mother Nature can throw at them.

Rhododendrons, often, are found as the evergreen understory of deciduous forests. This means when winter rolls around, they are exposed to a lot more light and a lot more wind than they had to endure during the remaining months of the year. The ‘droop and curl’, otherwise known as the most excellent word ‘thermonasty’, is the plants’ way of protecting themselves from two concerns:

1. Loss of Moisture. Protecting the more delicate and often less protected underside of a leaf, thermonasty, helps lessen moisture loss through relatively large leaves. Because rhododendrons are shallow-rooted they are even more susceptible to moisture loss than other broadleaves evergreens.
2. Too Much Light. A 2008 study on the Overwintering Leaves of Rhododendron illustrates another reason for the winter ‘droop and curl’ is to limit the leaves’ light exposure. Turns out that leaves of some rhododendrons left exposed in the winter actually causes a chemical reaction that results in damage to the plant. This has to do with the plants exposure to light and the lack of photosynthetic activity in the winter, which would normally regulate the chemical reactions happening in the plant. In other words, too much light in the winter = leaf damage.

Protected Rhododendrons in a forest exhibit thermonasty in response to the loss of canopy cover.

Rhododendron exhibiting thermonasty to a less severe extent in milder temperatures in Tuscarora State Park, PA

Some Rhododendrons and other broadleaved evergreens (like the Mountain Laurel) manage their light and wind exposure in different ways, through smaller leaf pores and more layers of the waxy coating on the leaves. In the case of all plants, the cuticle, or waxy coating, acts as a natural sunscreen. Broadleaf evergreens that do not exhibit thermonasty, have more of that protective layer. But in the case of our native rhododendron, specifically, Catawba Rhododendron (Rhododendron catawbiense), the thermonasty is all part of its natural health care routine.

Mountain Laurel, though living in the same environment, do not exhibit thermonasty to the extent our native rhododendrons do. They have other ways of dealing with winter extremes.

So, save your money and marvel at the wonders of nature because Mother Nature…she’s got this.

And…just in case you needed a reminder of spring:

Springtime Crocus and Tulips also exhibit thermonasty when they open with the sunrise in the morning and close as the sun sets.

Is there a time when Wilt-Pruf and other anti-dessicants are useful? Well, yes, when you are trying to fool Mother Nature. If you are trying to nurse a less-hardy plant along in your colder zone an anti-dessicant may help in this effort. Also, if you have purchased some of the left-over plants that didn’t sell when they were in full spring boom, or procrastinated in getting them planted until October, then an application of an anti-dessicant may help get a newly planted broadleaved tree or shrub through its first winter.