The topic we are about to discuss is a secret. You might want to lower the blinds and send the kids out to a movie before starting. It has, in the past, been considered cheating, and the funny mystic rites passed on to initiates in dribs and drabs. There was official sanction against it at truss shows, although we are now more enlightened and Rule X has been removed from the rulebook.
I speak, of course, of Winter Protection of Rhododendrons. OK, go ahead and laugh, but I am perfectly serious! Twenty years ago, Winter Protection was in the same no-no category that trimming leaves, leaf polish, and food coloring are in today. Why? Actually there was a good reason, believe it or not. Twenty years ago the members of the Massachusetts Chapter were just beginning to take the measure of our rhododendrons, trying many new and unknown varieties in our gardens and greenhouses. The rest of the rhododendron world was incredibly generous to us with both plants and advice, and while we were terribly grateful for the former, we soon found out that the latter often made no sense in our area. Plants that grew heartily when just "stuck in" often turned up their toes when given the T.L.C. recommended by the English or our West Coast friends. In particular we were most often warned that plants were "not hardy" for the East Coast. It was, it seemed, too cold for most rhododendrons in our neighborhood. (Even the famous Dr. David Leach made this mistake. If you consult his Rhododendrons of the World, you will see that he considered only a handful of species growable on the East Coast.) Because we wanted to grow rhododendrons out of doors, we became suspicious of any methods of ameliorating our New England weather. To say " yes, Mr. X grows that, but he protects it" would both sully the grower and condemn the variety. Gradually, as we became more familiar with our treasures and how to grow them in our uncertain weather, we have recognized winter protection for what it is - a very valuable tool for both the ambitious grower with irreplaceable rarities and the novice with his first few "tricky" varieties.
The techniques you will choose depend on three factors: the size of your plant, the current health of the plant, and which part of Winter you are trying to protect against. Some things can be done for just about any rhody, others are only suitable for special cases. Small plants are best treated en masse, in nursery beds, or otherwise herded together, as they have special needs and are also more vulnerable. All rhododendrons are hardier when mature than as babies.
For larger plants, there is a physical limit to protection; eventually you will have to come to grips with reality and let a plant either sink or swim. Reality usually sets in at about four feet. Health is an important factor too; a healthy plant is a hardy plant, or at least as hardy as it is gonna get. If you are going to try to sneak rhododendron rex past Ma Nature in Zone 6, you need to make it as happy and healthy as possible to have even a ghost of a chance. Which part of winter are you trying to defeat? If the plants are normally hardy in your area and you are simply giving them time to become established, that is fairly simple. If the plants are borderline hardy ones that just need a little help to get past the worst excesses of winter, that is also pretty easy in most cases. Perhaps you didn't get everything planted from the terrific end-of-season sale at your local rhody emporium and you need a place to stash them for the winter? Hey, no problem! But if you are thinking of a specimen plant of 'Dr. Calstocker' on your front lawn, you need a reality check, because Ma Nature will always be one step ahead of you. It's her job.
The simplest winter protection is a deep (four to six inches) fluffy mulch of whole or chopped (not ground!) oak leaves or salt marsh hay spread out over the root zone and snugged right up to the base of the plant. It is especially effective with seedlings and rooted cuttings, although it is useful for somewhat older plants as well. This mimics the protection that the mass of an older plant will give to its own root zone; it shades the soil and prevents rapid freezing and thawing, and it shades the stems on young plants to prevent bark split. This mulch can be applied right on top of your regular mulch as soon as the ground freezes, and should be taken away in the spring. If you are mulching a nursery bed full of young'uns from P4M, you can even tease a THIN layer of hay, or perhaps scatter some fresh pine needles over the leaves as well, which will give some protection from winter sun. This is not necessary if the beds have shading in the winter from high pines or from artificial sources, and in any case should never cover more than 50% of the leaf surface. This technique is contraindicated for large, dense, mature plants; the extra protection will only coax Mr. Vole to take up residence and you don't want that. Trust me on this.
Another good, easy technique for protecting a nursery bed is to erect either old-fashioned wooden snowfencing or lattice over the bed, like a cover, except that it is set eighteen inches to two feet above the ground - enough clearance to be able to reach in and check the plants or fluff up the mulch. For larger plants in a nursery row, I have seen snowfencing effective up to four feet above the ground; higher than that and the side wind tends to cancel out the benefits. The lattice can be removed in the summer, or left on all year for babies. You can think of this as a very tall mulch - it serves all of the same purposes, except to retain soil moisture, and works as well for medium sized plants as babies.
For hybridizers, a variation on this method can be very useful if you are using plants in your breeding program that are not quite hardy in your area. In a raised bed, plant your breeding stock with enough room to grow large enough to bud up. Now erect sides on top of your raised bed (you can use 1 X stock or outdoor plywood for this - it won't be in contact with the ground). Finish with snowfence or lattice on top. When you are done you will have a box, open at the top, but otherwise well protected from the weather.
There are two major problems with this setup; first, it is unsightly, and thus unsuited for high-profile spots in the garden, and second, it is very attractive to rodents and you will have to trap or bait inside the bed. This is particularly bad in good snow years; do not neglect this chore. I know it sounds like a lot of work, but I have known breeders to get a zone and a half extra this way. Does it go without saying that this is for plants that will bud up as reasonably compact plants - say, no more than 18 to twenty-four inches?
Perhaps your problem is not a nursery bed but a single plant in need of a little extra help? If the variety is one that you are not worried about the hardiness of, but just a plant planted late in the year, or maybe one whose top is not well balanced to its rootball, then the familiar "tent" made of burlap and stakes can be all that is needed. Sometimes in New England we suddenly get unusually dry, cold weather, often accompanied by cutting winds, and the frozen ground makes it difficult to put up a formal tent. If the plant you are protecting is small, it is easy to just lay a few pine boughs over it. But if you have a larger plant, two, three, or four feet tall, this obviously won't work. Instead, cut three or four young white pines about a foot taller than the plant to be protected, and lean them, teepee style, against the plant. Tie the top, arrange the branches for the best protection, and Presto! a tent that will shade and cut the wind until spring. Either of these "tents" will give you about half a zone of protection.
Of course the ultimate winter protection is a cold greenhouse, with nighttime temperatures maintained artificially with a heater. Drawbacks? Cost, for one: greenhouse, foundation, no-freeze water system, heater, cooling system, and the energy costs to run the thing. The less labor-intensive you make it, the more expensive it will be to build and run. How about a Quonset Hut? Less expensive than a true greenhouse, yes, but still pricey if all you want to protect is something smaller than a crop. In addition, you will still have to deal with overheating, temperature fluctuations, sunburn, and plant maintenance; many an amateur nurseryman has lost a crop by going on vacation in the winter, when nothing is happening.
So what we need is a place to keep a relatively small number of plants protected from the elements that will not cost an arm and a leg and will need a minimum of attention. What we need is a coldframe. Coldframes come in all sizes and shapes, but all are essentially the same thing - a pit dug in the ground, lined to shore up the sides so they don't cave in, and covered with a light-permeable lid with provision for ventilation and access for maintenance of the plants kept within. Properly built and maintained, they will last several lifetimes.
I have seen coldframes as big as barns; well, it was a barn, built over a pit twenty feet deep, with celestory windows for light and a big ramp for truck access. This overwintering facility for a large wholesale nursery provided the minimum of maintenance (mostly done by Ma Nature) with a maximum of flexibility, allowing plants to be shipped dormant over an extended period in the spring. If you have the good fortune to be given a tour of Wellesley College's greenhouses, don't miss their big walk-in coldframes. These were renovated about a decade ago by the Wellesley greenhouse staff to take advantage of advances in glazing technology, and they are now handsome units, clean and airy, striking envy in the hearts of their fellow gardeners.
Most of us, of course, don't need garage-sized coldframes but there is a bare minimum you must observe if it is to work for you. Simply put, by digging down into the earth you are tapping into the stored heat trapped in the earth at the end of the summer. Every day the sun shines, the glazing allows the frame to renew at least part of the heat released at night into the frame (the frame is opened a little every day so that the heat buildup is not too radical and harms the plants). Thus the bottom of the frame is the last place to freeze in the early winter and the first place to thaw in the early spring; in addition, temperature variations are evened out so there is no barksplit or heaving. To work, then, you need an air mass inside the frame that will be enough of a heat sink to perform this function without requiring such high temperatures that the plants will suffer. I saw brick lined frames in England that were no more than three feet by three feet, but of course, the last winter the English had was in 1962; they usually just have two Decembers and segue right to March. Here in New England, we need a bit more than that; I would say three-and-a-half feet wide by four to six feet long, and nine inches deep after backfill at a minimum.
The amount of protection such a frame will give depends on the materials the frame is made of, the glazing, and how deep the frame is dug. In my best coldframe, I kept an eighteen inch tall plant of Rhododendron edgeworthii, which went through several winters with minimum temperatures of between minus twelve and minus five degrees F. until voles made a nest out of it and killed it. I also had several "greenhouse" azaleas that were overwintered there and then brought in to the house in March to bloom. Neither the azaleas nor the rhody ever had any leaf damage or pip loss. Unfortunately that same extended family of Mr. Vole also reduced the azaleas to compost, giving me a hard lesson in vigilance or lack thereof. Rodent traps in coldframes should be checked at least twice a week, and bait stations checked and renewed every three weeks or so. Frames can be varmint-proofed, but it is expensive and a LOT of trouble.
Coldframe too much work? Or maybe you are going to Myrtle Beach for the month of January, and have no one to look after the frames in your absence? You could try a "Fred Frame" invented by our very own Fred Knippel. This is an extremely simple arrangement using some of the same principles as a coldframe, but it is more of a cold-storage technique, because once it is closed up, it cannot or should not be opened until spring. Choose a flat vegetationless spot at least six feet by ten feet, where water never collects in spring (this is important!). Using pressure-treated 2 by 12's, build a topless, bottomless "box" slightly less than four feet wide by eight feet long. Center it in your space, making sure that it sits firmly in contact with the ground. About mid-November in our area, fill the frame with your plants and water well. This will be their only watering all winter, so do it twice if you need to. Let the plants sit for a couple of days so the foliage will have a chance to dry. Then lay any tall plants over on their sides, and cover securely with a four by eight piece of outdoor plywood. You can use pressure-treated stuff but NOT new - let it weather before you use it. Do not forget Mr. Vole; leave bait at least, but a pan full of tomcat-used kitty litter has provided our best protection.
If we are still in Indian Summer mode when you set up your Fred Frame (the very best time to do it), you can leave it without its lid until colder weather comes. Eventually, though, the nights will get down to 25 degrees F. on a regular basis, and it will be time to put on the lid. Wait a week or two more now, until the ground freezes an inch or two, then cover top and sides with coarse woodchips. At my place in zone 6, we put six inches on top and about a foot on the sides. In colder zones, I think I would put more.
What is the theory behind this? It certainly allows for a slow cooldown and warmup, and keeps a nice even temperature during the winter. Does it keep the plants from freezing? I don't think so, although I do think it can't get too much below freezing because of the condition of the plants at the end of the winter, by which I mean near perfect. In any case, we are going to find out because I have acquired a remote sensing recording thermometer which will answer that question providing it will record through several inches of frozen woodchips. And frozen they will be, solid as the Rock, which is why I said it cannot be opened in winter. The exception to this is when we get a long January thaw with some decent open weather. The woodchips can then be shoveled off and the frame lid propped open or, better, taken off and the contents checked for rodent damage. Renew the bait and/or the kitty litter and reseal the frame. Do not wait for cold weather to return to shovel the chips back on. Try to get this all done in one day so that the plants (and the ground) hardly know that they have been disturbed. How much protection can you expect from this contraption? I honestly couldn't say, because we haven't reached its limit yet, but I would say that Zone 8 or so will probably be a practical limit (no citrus!).
Perhaps you have no beefy male to do your bidding? There is something else you can try. Although I have never personally done it, Blanchette Gardens in Carlisle, MA uses it to overwinter thousands of perennials with about a ninety percent success rate, depending on the winter. Once again, choose a flat unvegetated space where NO water will collect in winter or early spring, water your plants thoroughly, and let the foliage dry. Turn all the pots over on their sides, preferably in as compact and orderly a fashion as possible, and pointing in the same direction, biggest plants in the middle. Place rodent bait, traps, and/or kitty litter in several places. Cover first with one or more sheets of Microfoam, then with a stout single sheet of plastic (check for holes to avoid drips). The Microfoam and the topsheet must be large enough to cover the whole pile, with enough to spare to secure the sides with boards or bricks, or bury the edges under sand or woodchips. The best way is to use a combination of both techniques - boards or bricks to securely weight the edges against winter storms, sand or woodchips to completely seal the edges. Keep an eye out for rodent tunnels.
This technique will net you a half zone to maybe a zone of protection, and no guarantees of survival. Blanchette Gardens has used this for years, and although it is probably their best solution it still gives them mysterious die-offs despite the fact that nearly all of the plants are perfectly hardy.
There is one last thing I can describe. For many moons I have worked on the Chapter's exhibit for the MassHort spring show. If I have to be the one who holds the plants to be forced, I try to keep them in a deep coldframe or cold greenhouse so that they can be moved to the facility in Waltham no matter what the weather. One year our arrangements fell through at the last minute, and I got stuck holding the bag; one-and-a-half truckloads of mature, fully budded plants arrived at my place in the middle of a very raw November. Alas, there was no room in the inn! I was forced to find someplace out in the weather to keep them. They were much too big to put in a Fred Frame, and I really didn't trust the Microfoam.
The weather got progressively more miserable as I dithered over what to do, until Ma Nature stepped in. Bruce forecast a truly terrible Thanksgiving freeze; my exposed rootballs would be toast unless I did something P.D.Q. So I rounded up my long suffering hubby and we moved all of the plants under the shade of some very large white pines, and arranged them in a compact rectangle, grading them by size of rootball or pot. I started with the largest rootballs at one end and gradually got down to the smallest pots at the other end. Then the entire bunch were carefully covered up to their lower branches with coarse wood chips. This was a time-consuming job because I had to make sure no pockets of air were left between the pots; sometimes this required adding chips a trowel-full at a time. I finished with about a foot of chips all the way around.
In two days the temperature dropped like a rock, and only came up again after it had frozen the lot. Then it poured, soaking the pile with its captives, then dropped like a rock again. The whole bunch was one solid mass solidly welded to the ground. We had very bad weather all December that year, and when the time came to start moving plants into the greenhouse, we had to chop them out with a mattock. Plants were moved in all through January, and the weather continued to be miserable. BUT - we lost not a single plant to cold that year and we were unable to detect any loss of pips or buds, either. As an emergency method, it worked pretty well, but I wouldn't want to depend on it as a regular thing. As a caveat, it must be noted that we were not using any particularly tender plants that year, so as a method of gaining extra protection, I would not recommend it.
A few final thoughts on the subject. I am frequently asked about the anti-dessicant sprays that are sold to the uninitiated. Usually they are harmless, as long as you don't get any on the underside of the leaf, and they may even do some marginal good in a plant that was planted too close to winter to get its roots established. But usually I feel the same way about them that I do about SUV's. Many people justify getting them because they make them feel safer during winter driving; then this confidence leads to recklessness that puts them MORE at risk, not less (ask any state patrolman!). If you think you can bring your 'Dr. Calstocker' through the winter on your front lawn in Concord Massachusetts with anti-dessicants, think again.
I am also frequently asked if one could bring a hardy rhododendron through the winter by simply putting it outside during the day and bringing it in at night. "It's too late this year to plant that nice 'Duke of York' you sold us, the ground is frozen!" they say. "Couldn't we just move it outside during the day, and move it into the kitchen at night? Then we could plant it in spring when the weather gets better." Then they look at me with imploring faces and sad puppy-dog eyes. In a situation like that, I have no choice but to follow Nancy Reagan's advice; I smile and "Just Say No."