As the days shorten, and the nights turn cold, gardeners in cold zones start to worry about their plants. Will they survive the winter? Will they flower next year? There are lots of reasons why plants don’t survive in winter, or never flower, and when choosing what to grow, and how to grow it, knowing what these reasons are, and which plants they apply to, is valuable knowledge. Let’s take a look:
- The plant is not cold-hardy enough – this seems obvious, but some plants simply are not built for very low temperatures, and no matter what you do they won’t make it. This is why knowing your hardiness zone is so important, and once you do, pay attention to the coldest zone listed for the plant you want. It can be frustrating, but there is no point in growing a plant that only survives in zone 6, while you live in zone 4.
- The variety you chose is not cold-hardy enough – Many plants, from hydrangeas to camellias, have hardy and not-so-hardy varieties. If you, like most gardeners, want to ‘push the limits’ a bit on what you can grow, then you should always seek out the varieties of plants that are most likely to succeed in your area.
- They naturally brown in winter – some evergreens, like certain arborvitae, or some boxwoods, for example, turn brown or bronzy in winter quite naturally. As soon as the spring growth returns, they turn green and this is not really winter damage, just a built-in protection the plant has. If you want the greenest winter garden, then there are often particular varieties of these plants, like the Emerald Green Arborvitae, or Green Gem Boxwood, that don’t bronze. Usually, of you see ‘green’ in the variety name, you can reckon this is a form that doesn’t bronze much, so its pretty easy to choose the right variety – even so, check to be sure.
- They survive but never flower – this is one of the most common issues, where plants sprout in spring and seem to be healthy and vigorous, but flowers are never seen. There are at least two reasons, but a common one is that some plants flower on new shoots that appear in spring, and others flower on side shoots or terminal shoots from older, existing stems. It is common for plants in colder regions to die down to the ground, but because the soil temperature is warmer than the air, and there is often snow cover, the base of the plants stays warmer, survives, and re-sprouts. So while tea roses flower well in the coldest areas since they bloom on new stems from the base, many shrub and climbing roses don’t, despite healthy growth, because the older stems they need to flower on never make it through the winter. This difference in blooming also affects pruning, but that is a different story.
Hydrangeas are more complicated, as some types flower on older stems, and others on new ones. The forms of Hydrangea arborescens, like the well known ‘Annabelle’, flower on new stems, and they are easy to grow and bloom in colder zones. The forms of Hydrangea macrophylla, the blue or pink mophead hydrangea, mostly flower on side and terminal shoots from older stems, so with some exceptions, like Endless Summer, it is harder or impossible to see much flowering below zone 6, since the stems tend to die to the ground in winter. Since these two species look very similar to gardeners, it is no wonder this issue is so confusing.
- No blooms in years with late frost – this is a problem seen in everything from fruit trees to magnolia, forsythia, wisteria, and several other kinds of early flowering plants. These plants have two kinds of buds – leaf buds and flower buds. They flower early, on bare stems, and the flower buds, which are perfectly cold-hardy when dormant, are killed by just a couple of degrees of frost once they become active. When the months of cold has satisfied their dormancy, they begin to grow quickly in the first warm days. Then the weather turns cold again, and the buds die. The leaf buds stay safely dormant for several more weeks, so they don’t get damaged. You may not even notice the damaged flower buds, as they just brown and drop off, or they are hidden by the newly-emerging leaves. The result though is that you have a nice, leafy tree, but flowers appear only every few years, or almost never.
The solution is to look for varieties of the plants you want that flower just a little later, and so stay dormant longer and usually miss those pesky late frosts. Among the magnolias, look for trees with girl’s names, like ‘Anne’ or ‘Susan’, which were bred specifically to solve this problem. With other plants, look for the varieties that flower the latest, as these will be the most likely to be successful.
- They suffered from winter burn – this is what we call the damage seen in spring on many evergreens, where the foliage turns brown and crisp as soon as spring comes. It is caused by dryness at the roots, making it more difficult for the plants to keep their foliage full of water, especially when cold, dry winter winds are blowing. It is increased when the soil freezes hard around the roots, making it hard for water to enter the plant, even if the soil is damp. In colder zones, most of this damage can be prevented by watering deeply in the days before the ground freezes hard, as wetter soil freezes less. As well, mulching over the root zone keeps the soil warmer, so it doesn’t freeze so deeply, leaving more available water. This condition is worse in newly-planted bushes, as they have a limited root system, and no roots deep enough to be beneath the frozen parts of the soil.
- They are damaged by sun scorch – although this can look like winter burn, the cause is different. As well as foliage, bark can be affected, and some plants are much more prone than others. Dwarf Alberta Spruce is well-known for being sun scorched, and the trunks of deciduous magnolias, although the tree is perfect winter hardy, can have this kind of damage because their bark is thin. Lombardy Poplar is also prone to bark scorch just above the surface of the snow, for the same reason.
With sun scorch, the rapid warming on the south side of a plant when it is hit by the sun after a very cold night, can warm the cells enough to thaw them. When the sun drops below the horizon, temperatures fall rapidly, and the rapid cooling makes big ice crystals in the cells, which rupture the walls, killing them. Slower cooling makes small crystals, that fit comfortable inside the cells, and no damage is seen. If you have plants that are prone to this problem, screening from the sun will usually prevent it. Luckily buds will usually survive, and the branches won’t die, so once seen, you can take precautions the following winter. Bark damage too, if not extensive, will heal, so this condition is usually not fatal unless it keeps happening every year.