Common Questions, Tools & Techniques  |  March 10, 2021

What are Self Watering Planters and How Do They Work?

Self watering planters Greenery Unlimited's Self Watering Planters.

Taking the Guesswork Out of When to Water

Houseplants have a problem. They rely on human caregivers to provide water, light, nutrients, pest management-- in essence everything they need to survive. But humans are inconsistent, and it can be difficult to infer exactly how much water to give your plant at any given time. In fact, overwatering is the single biggest reason houseplants perish. 

Sub-irrigated or self-watering planters eliminate the guesswork of watering by allowing plants to water themselves. 


Self watering pot

Houseplants are for the most part tropical plants that have adapted to the low light understory of forested regions surrounding the equator. Because the weather in equatorial regions is more consistent than anywhere else on the planet, plants that we’ve come to term as “indoor plants” generally have one thing in common: they don’t like change.


Eliminating Inconsistency = Happy Houseplants

Providing consistent care to plants is grueling work. You can’t control exactly how much light they receive if you’re not exclusively using a grow light, so their water requirements are constantly shifting. Two cups of water one week might be too much or too little the next. And this is at the core of the problem facing house plants. They crave the one thing most of their caregivers are unable to provide: consistency. We get busy and forget to water one week, and the next overcompensate by giving the plant an “extra drink”. We then realize a week later that the soil is still very wet and we have overwatered, so we withhold water for another week, and a bad cycle emerges. Self watering planters eliminate the inconsistency in watering by allowing the plant to drink from a reservoir on an as needed basis. It also provides more consistent soil moisture for a longer period within the pot. As a caregiver, it simplifies knowing when to water as all you need to do is refill the reservoir when it is empty.


Refilling a self watering pot


Understanding Sub-irrigation

Sub-irrigation is a method by which plants are watered from below, instead of above. Our self watering planters use this method with a patented sub-irrigation insert that allows soil to maintain a consistent moisture level at the base of the pot, and for thirsty plants (looking at you Ficus family) the roots can grow through the insert and directly into the reservoir for continuous access to water and nutrients. 


Roots in a self watering planter


To understand the whole process, it can help to look to nature. Consider that when it rains the soil absorbs the moisture and gravity draws the water to the lower levels of soil and into clay and stone subsoil horizons. As the top layers of soil dry out, the plant's deep root systems are still able to draw water from the water reserves below in the subsoil on demand. The water retained in the subsoil allows plants to survive even in periods of drought as the roots still have access to water. Typical indoor planters don’t utilize this natural system, and instead force the plant to rely on top waterings when a human perceives the plant as thirsty. 

Self watering planters use sub-irrigation to deliver water directly to plant roots, without any guess work. The water reservoir at the bottom of the planter allows the plant to drink at its own pace and visually shows caregivers when it is time to water with an empty reservoir. 

The Science

The science behind why sub irrigation and self watering planters work comes down to some basic botany. Plants absorb water through osmosis and move it throughout the plant using capillary action and water potential differences. This whole process is called transpiration. When roots don’t have regular contact with water, they can’t effectively draw enough water to retain the rigidity of plants which is why you may see your peace lily drooping when it gets too dry. As any experienced plant parent can tell you, there are many environmental factors that go into how fast or slow a plant is transpiring and using water (light, heat, humidity are just a few). This is why sub-irrigation is truly the easiest way to ensure plants have enough access to water-- the reservoir allows them to process water as needed. On darker, damper days the plant will require less water than on hot, dry days. By having access to the reservoir, the plant doesn't need to rely on a human care giver to make incremental changes in watering to compensate for changing environmental conditions. 

So Does Every Plant Want a Self Watering Pot?

In our experience, mostly yes. The vast majority of tropical plant species prefer consistent levels of moisture in the soil and access to a water reservoir so that they have control over the rate of transpiration. Even plants with shallow root systems like cacti and succulents can be very successful in our self watering planter as long as they are receiving adequate light. Our patented insert in combination with aerations stones offer excellent root aeration while also drawing water up into the lower soil layer which mimics natural environmental conditions, even in desert landscapes. 

Self watering planter

Wet Feet, Dry Ankles

A saying we have around our horticulture department is that most tropical plants enjoy wet feet but dry ankles. They want to have access to water at the root level, but don’t want all the soil in the pot to be saturated. Most plants prefer the top few inches of soil on the dry said, and only want to experience moisture at the direct root level. The single most common reason people kill their plants is over watering -- wet ankles and a lack of air at the roots. A well-designed self watering planter will have a separation between the water reservoir and the growing medium. To ensure water is able to travel to the "feet" or lower root zone, a wicking material is used, like aeration stones that both absorb water and create air pockets within the lower soil layers.


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