Summer heat presents a unique challenge to cannabis cultivators, requiring a significant shift in irrigation strategy. High temperatures and intense sunlight dramatically increase the plant’s internal water usage and external water loss from the growing medium. This heightened demand for moisture is driven by the plant’s need to cool itself through rapid transpiration, which can quickly lead to dehydration and stress if not managed correctly. Understanding how environmental conditions accelerate water consumption is foundational to maintaining plant health.
Environmental and Plant Factors Changing Watering Frequency
The frequency with which a cannabis plant requires water in summer is determined by surrounding atmospheric conditions and the plant’s biology. A major driver is the Vapor Pressure Deficit (VPD), which quantifies the difference between moisture inside the leaf and the surrounding air. High summer temperatures paired with low relative humidity create a high VPD. This acts like a strong atmospheric vacuum, rapidly pulling water vapor from the leaves, forcing the plant to increase transpiration to regulate its temperature and draw more water from the roots.
A plant’s stage of development dictates its total metabolic demand. A mature plant in the flowering stage will consume significantly more water than a small plant in the vegetative stage. Flowering cannabis requires a high VPD, typically 1.0 to 1.5 kilopascals (kPa), to encourage vigorous transpiration and nutrient uptake. This maximizes the plant’s water usage to support flower development.
The size of the container and the type of growing medium heavily influence how quickly water is depleted. Plants in smaller pots have a limited volume of medium, causing them to dry out much faster than those in larger pots. Different mediums also have different retention properties; soil-based mediums hold moisture longer and require less frequent watering than inert mediums like coco coir. For example, a plant in a three-gallon container of coco coir may require water daily, while a similar plant in a larger soil pot might only need it every two to four days.
Practical Methods for Determining When to Water
Establishing a fixed watering schedule in summer is risky because water consumption changes daily based on weather. Growers must rely on empirical methods to assess the medium’s condition. The most reliable method for container-grown plants is the “lift test,” which involves physically weighing the pot to gauge moisture content. Growers should lift a fully saturated pot to feel its maximum weight and compare it to the weight of a dry pot. Watering should occur when the pot feels noticeably light, indicating the medium is ready for replenishment.
A simpler, though less precise, technique is the “finger test.” The grower inserts a finger one to two inches deep into the medium near the edge of the container. If the medium feels dry at this depth, it is time to water; if it still feels cool or damp, watering should be postponed. The goal is to allow the top layer to dry out to prevent fungus gnats and encourage root exploration.
Growers can also use simple moisture meters, which provide a consistent, objective measurement of the medium’s saturation level. These meters eliminate guesswork, especially in larger containers where the top layer may be dry while the root zone remains saturated. The meter should be inserted deep into the medium, away from the base of the plant, to measure moisture content in the main root zone. Consistent use of these methods helps establish a dry-down cycle, allowing the medium to partially dry before re-watering, which ensures the roots receive adequate oxygen.
Optimizing Irrigation Technique for Summer Heat
When watering in summer, the technique of application is as important as the timing, especially since high temperatures compound the risk of heat stress. The most effective approach is deep saturation, applying water slowly and evenly until 15 to 20 percent runoff drains from the bottom of the container. This ensures the entire root zone is saturated, washing away accumulated mineral salts and encouraging deep root growth. Shallow, frequent watering should be avoided because it trains roots to stay near the surface, making the plant vulnerable to heat and rapid drying.
The best time of day to water cannabis during summer is early morning, just as the lights come on or the sun rises. Watering at this time allows the plant to absorb and transpire moisture throughout the day, preparing it to handle the peak midday heat. Watering should be avoided during the hottest part of the day, as this leads to excessive water loss through evaporation and does not provide the plant enough time to prepare for the high-demand period.
An important factor in summer irrigation is the temperature of the water itself. The ideal temperature for irrigation water is between 68°F and 73°F (20°C and 23°C). Water exceeding 73°F holds less dissolved oxygen, which can suffocate roots, promote anaerobic bacteria, and impair nutrient absorption. Conversely, very cold water can shock the roots, temporarily reducing metabolic function. Observing the plant’s response provides feedback: a plant that droops immediately after being watered and remains heavy is likely overwatered, while a plant that droops just before watering and feels light is likely underwatered.