Nothing you put around a rhododendron will significantly moderate the cold temperatures you're likely to experience in a Wisconsin winter. What you do want to do is protect the plant from the freeze drying effects of sun and wind. Good old burlap attached to three or four stakes driven into the ground around the plant will accomplish this at a much lower cost and probably greater effectiveness than a plant protector bag. Just don't let the burlap actually touch the leaves. A tepee shape is useful for shedding snow.

The golden rule when protecting a plant is to leave the top open or use a porous material to avoid creating a greenhouse effect. Wind protection and sun protection are good, especially on younger plants. A good mulch layer is important also. Do not let the mulch come closer than an inch or two from the stem. That is to prevent vole damage and bark split.

Here is a link that might be useful: How to care for rhododendrons

Has anyone used a product called "Wilt Pruf" on evergreens to protect them in the winter? We used to use it at my hort tech school in a cold zone 5, and it seemed to work fine. Only problem is that it needs to be re-applied in mid-winter, and you need a warm, sunny day to do it, if I remember correctly. I had two well grown R. catawbiense at my house in south-central NH. We would often get stretches of intense cold where for a week or ten days night temps would reach -20 or colder and days not get warmer than 5-15 degrees. These plants were growing in full sun and pretty much in the wind--the leaves would roll up tight to reduce moisture loss, and they got through fine and bloomed profusely every year. I didn't provide any real protection because the previous owners hadn't, and these were 10-15 foot high specimens. I guess the point here is that once rhodies of the "ironclad" tribe are well established they can tolerate a lot.

Bruce

Yes, I have used anti-desiccant/anti-transpirant sprays such as Wilt Pruf and you are right that they need to be applied once or twice in mid-winter. They are also excellent to protect a plant when transplanting. Plants loose moisture from the bottom of their leaves through transpiration from openings called stomata, so it is important to spray the under sides of the leaves.

Most rhododendrons have natural protection from cold in that they curl their leaves to protect the stomata from exposure. However, sun and wind can be more than a match for rhododendrons when the ground is frozen. It is helpful to protect young plants since they are less resilient their first winter or two. Some azaleas take three winters to achieve their full cold-hardiness.

I'm a little surprised there has not already been a post citing lab studies which "prove" WiltPruf and similar products do not work.

I completely agree with rhodyman. I do use WiltPruf and many years of actual experience by me and others have convinced me that it definitely helps the large leafed rhododendrons come through the winter in much better shape, at least here in the Northeast. That does not mean it will help a Z7 plant survive in Z5, but it will help limit the damage to the leaves.

How to explain the lab results? I don't doubt the results or the scientific rigor of the experiments. I just think the ones I've read fail to measure what actually takes place outside in winter.

maingrower, I haven't seen any references about it not working, just about its limitations.

Here is a report from the University of Florida of an experiment testing it for transplanting.

Sixty-four 4-inch caliper live oaks (Quercus vir- giniana Mill.) were transplanted in Aug. 1984 from 1 area of a tree nursery in Oak Hill, Fla. to an adjacent area using a 44-inch tree spade. Trees were either sprayed with water, Wilt-Pruf (1:10 dilution), or Cloud Cover (1:2 dilution) the day before transplanting, or 6 oz of Terra-Sorb (water-absorbing, starch polymer) was added to the sides of the planting hole. Thirty-two trees were irrigated with 18 gal of water daily and 32 trees were irrigated 7 days after a 1-inch rain or the last irrigation (weekly). There was no interaction of irrigation rate with other treatments on survival of the live oaks. Fifty-five % of trees watered weekly and 65% of trees watered daily survived. Visual ratings after 21 and 37 days were greater if trees were watered daily than those watered weekly.

67% of trees survived when sprayed with water
80% of trees survived when sprayed with Wilt-Pruf
38% of trees survived when sprayed with Cloud Cover
54% of trees survived when sprayed with Terra-Sorb

Sixty years ago Wilt Pruf was discovered and became the first anti-transpirant on the market. It enabled nurserymen to transplant shrubs and trees any time of year without fear of excessive transpiration which could cause plant failure.

Over the years additional uses have been discovered:

1) protection against drying out when evergreen roots were frozen in wintertime, as well as during periods of drought.

2) effective on cut Christmas trees, wreaths, and greens to retard needle drop by keeping existing moisture in the foliage.

3) indoor plants could survive longer periods without watering by treatment with an anti-transpirant.

4) ornamentals exposed to excessive warm or cold winds were protected from drying out by spraying with an anti-transpirant.

5) storage of tubers, bulbs, and bare root stock was more successful when they were first dipped in an anti-transpirant solution.

One thing I found interesting, Wilt-Pruf is a natural substance obtained from Pine trees.

Here is a link that might be useful: University of Florida 1984 study of Wilt Pruf