Protect Evergreens Like Bamboo From The Cold, and Try Our Trash Can Trick
Why Are Evergreen Plants Less Tolerant Of Extended Cold?Evergreen plants are a little different from most plants when it comes to winter hardiness and typically evergreen plants are more easily affected by long periods of cold much more than deciduous plants. When the leaves remain on the plant through the winter they continue to pull water from the roots and will actually photosynthesize on warm days - an obvious advantage over deciduous plants but one with increased risk. Cold dry winds will quickly evaporate water out of the leaves, especially because the humidity is often very low in the cold of winter. When the ground freezes solid the roots are not able to pull any water from the soil, but continue to supply water to the leaves and so deplete the water stores within the root cells.
If this process lasts for too long then first the leaf tissue will be damaged followed by root tissue. Damage to the roots is obviously not good for your plants is the main reason you see sluggish growth following a cold winter. Extended freezing can bring an evergreen plant's hardiness level up a full ten or fifteen degrees, so an evergreen might be similarly damaged by three days of lows around -20 degrees or four weeks straight of -10 degrees. The main protection for evergreen plants comes from wind protection, which you can achieve using our trash can trick. The other most important protection is to make sure your plants are well watered before the cold weather sets in so your plant doesn't start out dehydrated.
Protect Your Plants Using Our Trash Can TrickTypically most plants will be the most easily damaged by severe cold when they are younger as fully established plants have harder tissues and the thick foliage acts to insulate the entire plant. If the weather forecast predicts weather colder than our average annual low, we run out and put an empty trash can upside down over all of our young plants. We only use the trash can trick for plants which are marginally hardy for the predicted weather, if plants are listed as being far more hardy then we don't worry about it. The reason that the trash can works so well is that it protects the plant from drying winds, plus the few extra degrees of insulation.
Another factor to keep in mind is that wind quickly cools or warms plant tissue, which can result in frequent freezing and thawing. The continued freezing and thawing and re-freezing can be much more damaging to plant tissue than just cold alone. The extra insulation actually helps by helping to keep the plants frozen during the day until the average temperature moves back into more normal ranges. You can leave the trash can on the plant for a few weeks. If you also have some large cardboard boxes, those work just as well. As plants age and grow their winter hardiness increases with the size of the plant, usually a good rule of thumb is that a plant is too big to fit into a large trash can then it probably doesn't need it.
Give Extra Protection To Plants Grown In ContainersRemember that plants grown in containers are far less winter hardy than plants grown in the ground. You should expect to lose at least one hardiness zone by growing plants in containers, so if you are in zone 6 then you should find plants that are hardy to at least zone 5 for container growing. And again, plants that are too dry before extended freezing will suffer more winter damage and this is more likely to happen with plants grown in containers. Check to make sure the soil is moist periodically, especially before long cold spells.
Of course, potted plants can easily have the opposite problem of being too wet in the winter, and especially for deciduous plants or perennials this can be a big killer. Cold and overly wet conditions will cause root rot so it's important to use a well drained potting mix. We have a great recipe that we use for our own plants here: Potting Mix Recipe. Using well drained potting soil and planting in containers that have sufficient drainage holes in the bottom go a long way in keeping plants from getting over-watered, and when it rains hard it's difficult to control how much water your plants get otherwise. Mixes that use sand, grit, or gravel actually tend to stay too wet in the winter even though they dry easily in the summer so avoid adding any of those to your mix. Adding more than 20% compost, peat, and/or coco coir can make your soil hold too much water as well. In all the years we have experimented with potting soils time and time again mixes that use shredded bark have the best overwintering success by far.
Besides keeping plants watered correctly, one of the most important ways to keep potted plants protected is to group all of them together into one clump. They will help insulate each other and reduce the freeze-thaw-freeze-thaw process that can damage roots. If you have plants on your patio, move them so they are right up against your house as this increases the temperature a bit. Some people even take the effort to wrap each pot with insulating material, such as many layers of cardboard. This works really well if you only have a few plants and is great when they are too big or there aren't enough of them to form a good potted plant clump.
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