Bottom watering is a technique where potted plants absorb moisture from the base upward through capillary action when placed in a tray or saucer of water. This method ensures the entire root ball is saturated, encouraging stronger, deeper root growth. Because the soil is thoroughly saturated, the timing for the next watering must be carefully considered.
The Mechanics of Bottom Watering
To perform bottom watering, place the potted plant, which must have drainage holes, into a basin, tub, or deep saucer. Fill the container with several inches of water, ensuring the water level covers a significant portion of the pot’s bottom half. The dry soil acts like a sponge, drawing water upward against gravity through the microscopic spaces between soil particles.
The duration of the soak is not fixed but depends on the soil type and pot size. Generally, the plant should remain in the water for a period ranging from 10 minutes up to an hour. The soak is complete when the top layer of soil becomes visibly moist to the touch, indicating the water has successfully wicked throughout the entire substrate. After the soil surface is damp, remove the plant from the basin and allow any excess water to drain completely from the drainage holes before returning it to its normal location.
Indicators for Watering Frequency
The frequency of bottom watering should be dictated by the plant’s current state of dryness. The most reliable method to gauge the need for water is the “finger test,” where a finger is inserted approximately one to two inches into the soil. If the soil feels dry at this depth, it is time for a thorough bottom soak.
A complementary method is the “weight test.” Lift the pot before watering to establish a baseline for its weight when dry. Water adds significant mass, so a pot that feels surprisingly light indicates that most of the moisture has been utilized or evaporated.
Waiting until the root mass is dry encourages a healthy wet-dry cycle, which is important for gas exchange in the soil. The roots need oxygen as much as they need water, and constantly saturated soil prevents this necessary exchange, leading to anaerobic conditions and root decay. For most common houseplants, wait until the top two to three inches of soil are dry before initiating the next bottom watering.
Environmental and Plant Variables
The ideal interval between bottom waterings is influenced by plant-specific needs and environmental factors. Plant species dictate the necessary level of soil dryness before rehydration. Succulents and cacti require their soil to dry out completely between waterings. Conversely, tropical foliage plants, such as ferns or peace lilies, prefer consistently moist conditions and need water more frequently.
The material of the pot affects the rate of water loss. Terracotta pots are porous, allowing water vapor to pass through the clay walls, causing the soil to dry out faster. In contrast, non-porous containers like plastic or glazed ceramic retain moisture for longer periods.
Seasonal changes play a role in a plant’s water usage. During the winter months, many houseplants enter a period of reduced activity or semi-dormancy, requiring less water. Conversely, the high heat and increased light intensity of summer and active growth periods accelerate transpiration, necessitating a shorter interval between soaks.
Local environmental conditions alter the frequency. Plants situated near heat registers or in low-humidity environments lose water more rapidly than those in cooler, more humid settings. Higher light exposure also increases the rate of photosynthesis and transpiration, pulling more water from the soil.
Addressing Mineral and Salt Accumulation
Bottom watering leads to the accumulation of dissolved solids, such as mineral salts from fertilizer and tap water. Because water is absorbed from the base and evaporates from the topsoil, these substances are drawn upward and left behind as white crusts on the soil surface. Over time, this buildup can reach toxic levels, interfering with the plant’s ability to absorb water and nutrients.
To prevent this concentration, flush the soil periodically with a thorough top watering. This process, known as leaching, involves pouring a large volume of plain water over the soil surface until it drains freely from the bottom. This washes the accumulated salts downward and out of the drainage holes.
Performing this leaching flush every four to six bottom watering cycles is a good preventative measure for most plants. Signs that the buildup has become severe include a noticeable white crust on the soil and the appearance of scorched or burnt-looking leaf tips and margins, which is a common symptom of salt toxicity.