You may not think your plants need to be watered in the winter. Usually, we think of plants wilting with hot dry weather in the summer. But winter can actually be just as damaging, with very low humidity and cold winds. Think about how your skin is much more dry and prone to cracking in the winter compared to the summer, especially if you spend a lot of time outside. This year, it is even more imperative to make sure your landscape is protected because we have had very limited rainfall all year, and the water table is low, which means plants aren’t able to draw up much moisture from deeper in the ground.
To understand why the winter can be very hard on plants and how proper watering can provide protection from the cold, we need to understand how water affects plants from the inside. We’ll keep it pretty basic, since not everyone is interested in a plant biology lecture 😉 . There are a great number of mechanisms that plants have developed through evolution to protect themselves from the cold, but we’re going to focus on just a couple.
Starting with two basic groups of woody plants, we have deciduous plants and evergreen plants (bear in mind this is more of a spectrum than two separate categories, and some plants may fall in the middle). Deciduous plants protect themselves from the cold by dropping their leaves and pulling the free liquid sap into the root system, where it is insulated by the ground. This prevents the sap from freezing, which would damage the plants at a cellular and more macro level, such as splitting the bark through expansion during the freezing process. However, in cases of extremely cold and prolonged dry weather, the twigs and buds can be dried out or damaged to the point that the buds and greenwood are killed. This causes dieback of the branches so that when the tree or shrub leafs out in the spring, the ends of the branches will be dead. In an established plant, it is usually not fatal, but in a newly planted tree or shrub, the root system may not be large enough to regrow the plant in the spring, which would result in death. In addition, many plants are actively growing their root systems in the winter, and if they don’t have enough water, it limits the growth of the roots. This may cause the plant to suffer the following summer, when the plant cannot draw enough water from the soil to replace the water lost through evapotranspiration, which again, may result in death.
Evergreen plants protect themselves by increasing the amount of dissolved substances in the sap of the plant that lowers the freezing point of the liquid. This works the same way that ocean water doesn’t freeze. Think about how different pine resin is compared to maple sap (which can be cooked down to form maple syrup)—pine resin is very thick, dark, sticky, and smells very strong, while maple sap is thin, watery, almost clear, and has a light, sweet scent. Just by looking at these two substances, you can guess that pine resin has a lot more substances in its sap than maple sap does, and that is water gives it a lower freezing point. It would have to get very, very cold before a pine tree would be damaged by cold weather. However, a pine tree does still need water to carry out cellular functions. If a pine tree does not have enough water, it can still dry out and die.
Herbaceous plants are plants that do not grow bark and typically die back to the ground each year. Perennials and grasses are considered herbaceous. Herbaceous plants draw their sap into the root system or a storage organ, such as a bulb or a rhizome, and the portion above ground is allowed to die, which typically occurs during the first hard frost of the season. Some herbaceous plants can survive prolonged dry periods, while others need consistent moisture during the winter, depending on the climate where they originated. Some bulbs that come from arid climates, such as the Mediterranean or South Africa, will actually rot during wet winters. However, most perennials and grasses that we grow in NC need consistent moisture in the winter to survive. Otherwise, the crown may be damaged by cold, or the storage organ could dry out and die.
So why do plants dry out in cold weather? The main reason is the humidity in the air is very low, because cold air cannot hold as much moisture as warm air can, and humidity is actually one of the main things that helps keep plants from drying out. What it all boils down to is that plants have to lose water into the air in order to pull more water in from the soil. Deciduous trees lose some water through their bark even when the leaves are dropped. Thus, if they do not receive enough water in the ground, they will eventually dry out due to water loss through the evergreen leaves, bark or even roots. The lower the humidity, the faster water is lost.
Adding wind into the mix makes winter damage even worse, leading to a problem in the landscape known as windburn, where evergreen leaves may turn brown due to the cells dying from becoming completely dried out, and this can occur even with adequate soil moisture in very windy locations. A less serious condition called bronzing may occur on some evergreen plants, particularly Korean boxwoods, where the leaves don’t die, but the chlorophyll is damaged or reduced and the other plant pigments are visible. Bronzing is a sign of stress, but if the plants are receiving adequate water, they will recover in the spring.
Thank you for sticking it out through this long post! I hope you have a better understanding now of why it is very important to keep plants watered through the winter. Most winters, rainfall and snow will provide enough moisture, but this year your landscape may need some extra help, especially if you have plants that were planted earlier in the year. Check the soil by feeling the top few inches of soil using your fingers or a soil probe. If the soil is dry, water the plant until the top few inches of soil are well soaked. A good rule of thumb is to provide 10 gallons of water per week to a new tree or large shrub, and an inch of water per week to perennial or small shrub plantings. Good luck!