Often, in the early stages of a party, guests sheepishly mill about the room, introducing themselves and their jobs in the hope of finding safe conversational harbor. After I say I study agricultural science, I notice a concentrated look in some people’s eyes, seemingly internally writing a note as a reminder. Later, they draw me aside and with downcast eyes and in a hushed tone, the sad fate of a recently deceased houseplant tumbles out. Their tortured confession is followed by a flood of questions in their effort to salvage meaning behind their plant’s all too short existence. For the vast majority, I determine that the root cause (he-he) of death was probably by overwatering. This diagnosis always sparks wild looks, frustrated gasps and much hand wringing, after all, this time they had watered their plant every morning with earnest dedication. How hard could it be after all, IT’S JUST A PLANT (and an ungrateful one at that)!? To help out, I have compiled a few rules of thumb, with a following longer explanation, for guidance.

My Rules of Thumb

• Every plant needs water.
• Each plant is adapted, based on their native habitat, to different moisture levels, for example, cactus grows in the desert, and therefore requires little moisture. It’s always a good idea to read the label to get an idea about origin and follow the recommended watering regime.
• Plants can’t move when the going gets tough. As a result, they are incredibly flexible to changes in environment, and can tolerate a relatively wide range of conditions. Within limits, periods of drought and waterlogging are tolerable. But, stress, just like in people, weakens the plant and can lead to disease.
• The pot used should have holes at the bottom.
• Slowly water the soil around the plant (avoid leaves) until water comes out the bottom. Stop.
• After a few days (in the case of the anxious, every day), put your finger a few inches deep in the soil. If the soil is dry, water as described above. If not, wait longer. Seldom deep watering is preferable to shallow, frequent watering.
• EXCEPTIONS!!!
• To germinate seeds, or establish seedlings the soil should be constantly moist.
• The watering regimen should be adapted to take into account both the type of plant, and the characteristics of the soil.

First, some soil science
To understand these rules of thumb, it’s necessary to learn some basic soil science. Although there are several different mediums that can be used to grow a plant, we will exclusively deal with soil. Its structure is defined as the arrangement of aggregates composed mainly of three components: sand, silt, and clay particles. Between these aggregates is space, or soil pores, filled with either air or water.

How the aggregates are positioned in relation to each other determines the movement of air and water through the soil. In an evenly textured soil (a proportionally balanced amount of soil, sand, and clay- known as ‘loam’) up to 50% of its volume is pore space. Below is the classification used for soil pores:

• Macropores: large sized pores, are greater than 75 μm. Due to their large size, water and air can enter and leave easily. These are the spaces roots can grow in as well. Macropores are essential for drainage, aeration, evaporation and gas exchange.
• Mesopores: medium sized pores, between 30-75 μm, are essential for capillary water distribution and water storage.
• Micropores: small sized pores, between 5-30 μm are water storage sites. Pores smaller than this diameter range can’t be tapped by the plant.

Mesopores serve also as a spatial connection between micropores and macropores, without them, water is unable to infiltrate the soil. As a result, an ideal soil texture has an even mix of all three.

How soil pores relate to water
Repurposing the concept of soil pores for plants, the amount and size of soil pores determine the availability of water to the plant. Agronomists classify water availability to the plant, which is determined by soil structure, with three levels of decreasing availability: ‘Saturation’, ‘Field Capacity’, and ‘Permanent Wilting Point’.

‘Saturation’ is the term used to describe when all soil pores are filled with water, and therefore no air is present. This is confirmed by a simple field test by squeezing a handful of soil, resulting in a trickle of water. Roots require air for respiration, and without specialized structures to bring air to the roots, such as hollow stems found in rice, they die, killing the point as well. Depending on the soil texture (relative proportion of clay, silt, and sand), once the rain or irrigation has stopped, a field will take from a few hours to days to drain.

When free drainage has stopped, the soil has reach ‘Field Capacity’, which means the macropores are again a mix of water and air, and the mesopores and micropores are full of water.

Without any further watering, the combination of evaporation, and plants using the water through transpiration, dries out the soil. The remaining water in the smallest of pores is held more tightly by aggregates, making it harder for roots to extract. At the ‘Permanent wilting point’ water is only present in smallest pores and entirely unavailable to plants. Without water, plants lose turgor and wilt. Water stress is also indicated by yellow leaves.

Therefore…
Watering until water drains out of the bottom of the flowerpot means the soil is reaching field capacity, ideal as both water and air are readily available. The holes in the pot allow for water to freely drain, otherwise it collects at the base of the pot, potentially leading to root rot. Any water in the soil can be felt from putting a finger in the soil, compressing it, and squeezing any water out of the soil pores. If it feels dry, the soil is likely near the permanent wilting point and needs to be refilled to field capacity.

But, but, but
Based on the above, why isn’t watering every day, theoretically maintaining field capacity (as long as the bottom drains) considered best practice? Time and texture. Soil texture is the composition of a soil of either sand, silt, or clay. They refer to particle size and not chemical composition, sand is the largest particle, followed by silt and then clay.

The larger the particle, the less able it is to hold on to water. In a sandy soil, water infiltrates quickly, whereas in a clay soil tightly it can take twice as long. The point when water leaves the bottom of the pot is only the largest pores draining, but the smaller macropores require longer. A sandy soil, which mostly has large macropores, reaches field capacity (and then permanent wilting point) quickly, but a silt or clay soil needs significantly longer. As a result, in a clay or silt soil, water added every day could be before field capacity has been reached, leading to saturation. Rather than risk waterlogging, it’s best to let the soil dry out until near the permanent wilting point, with the added benefit of forcing greater root development as the plant tries to use all of the soil water reserves.