Winter presents a unique challenge for the care of outdoor potted plants. The primary goal is to prevent the roots from drying out (desiccation) while simultaneously avoiding root rot and damage caused by freezing. Unlike plants in the ground, container plants have a small, exposed volume of soil that freezes and thaws quickly. Watering must be based on the plant’s true need, not a fixed schedule, to sustain minimal metabolic functions without creating a lethal ice block around the roots.
The Critical Difference: Plant Dormancy and Water Needs
The reason watering frequency drops dramatically in cold weather is tied directly to the plant’s biological state of dormancy. Dormancy is a natural survival strategy where the plant significantly reduces its metabolic activity in response to shorter daylight hours and cooler temperatures. This reduction means the plant’s internal processes, including growth and water uptake, slow down considerably.
Transpiration, the process where the plant loses water vapor through its leaves, is also greatly diminished, which lowers the demand for water from the roots. However, this water need is not zero, especially for evergreens that retain their leaves or needles throughout the winter. These plants continue to lose small amounts of moisture from their foliage on sunny or windy days, making them particularly vulnerable to desiccation when the soil is frozen solid. Providing a small amount of water during a thaw prevents the fine root hairs from drying out and dying off.
Determining When to Water
Establishing the necessity for water must be done by directly assessing the soil moisture, rather than following a calendar schedule. The most reliable assessment technique is the “finger test,” which requires pushing a finger two to three inches deep into the potting mix. If the soil feels dry at that depth, the plant may need water, provided the soil is not frozen solid.
Another simple method is the “lift test,” where you assess the weight of the container. A pot that feels surprisingly light indicates that the soil has dried out significantly. Never water a container that is frozen, as the plant cannot absorb the water and the added moisture will expand and potentially crack the pot or damage the roots. Wait for a period of thawing before performing the moisture assessment and watering if necessary.
Key Variables That Dictate Frequency
Several external and structural factors modify how quickly a container will dry out, which directly influences the timing of the soil moisture assessment.
Ambient temperature swings and the resulting freeze-thaw cycles can accelerate moisture loss. When temperatures fluctuate, the soil surface can dry out even if the air is cold, and the roots remain unable to draw water when the soil is frozen.
Wind exposure is a major contributor to plant desiccation, as air currents wick moisture away from the soil surface and the plant’s foliage. Even in cold weather, a persistent wind can dry a plant out more rapidly than a warm, calm day.
The material of the container also plays a role. Porous materials like unglazed terracotta allow water to evaporate through the pot walls, causing the soil to dry faster than in plastic or glazed pots.
Executing the Winter Water Session
Once the soil assessment determines that watering is needed, the process must be executed carefully to avoid root damage from freezing. The ideal time to water is on a mild, sunny day when the air temperature is above 40°F (4°C). Watering should be completed early in the day, preferably before noon.
This timing allows any excess moisture to drain away and gives the soil time to absorb the water before evening temperatures drop below freezing, minimizing the risk of the root ball turning into a solid block of ice. When watering, apply tepid water to avoid shocking the plant’s root system.
Water deeply until you see runoff escaping from the drainage holes at the bottom of the pot. Ensuring excellent drainage is paramount, as standing water in the saucer or at the bottom of the container will freeze and cause root rot, which is often fatal to the plant.