What is a self-watering reservoir and do I need one?

Adding a water reservoir to the pot before planting up has a number of benefits, but is not suitable in all situations. 

A self-watering reservoir is a plastic tank installed in a plant container underneath the plant/s and filled through a tube at the surface.  Rather than being watered from the top, the plant takes up water from the reservoir through its root system. 

There are some inherent benefits to plants of watering this way.  They can take up water as needed, so there’s less risk of over or under-watering, and it is generally the case that watering from beneath decreases the chance of fungus and disease. But reservoirs are usually put in not for plant health but for convenience – here’s why:

1. A reservoir allows you to water less frequently

A tank will hold from 3 litres of water to 24 litres for the largest size, and so you can go for longer periods without watering, handy for holidays. Handy too for pots that are right at the bottom of the garden or just a bit inconvenient for watering constantly. There is a float in the filler tube that indicates the water level in the tank.  Obviously the water will be used more quickly in warm weather, but you may find it does not need topping up at all in the winter or particularly wet periods. Diluted liquid feed can be added to the reservoir in order to give a steady supply of nutrients.

2. Use and waste less water

When watering with a can or hose, there can be quite a lot of wastage.  Dry soil holds little water, so if a pot has dried out, water will tend to drain straight through, wetting the soil only on the surface.  A reservoir ensures water is used as efficiently as possible, and keeps the soil around the roots constantly just damp enough

3. Reduce run-off from the base of the pot

As above, watering in the normal way will invariably mean water runs out through the drain holes in the base of the planter.  If it sits on a light coloured paver or tile this can be unsightly.  While using a reservoir does not entirely cut out run-off - a rainstorm may still soak the compost right through – it will significantly reduce it.

4. Keep indoor plants healthy

It is very tricky to plant directly into an indoor plant pot and get the level of watering right.  The temptation is to overwater, which results in waterlogged soil at the bottom of the plot and usually a dead plant! Here too the reservoir is your friend, as the plant will access just the amount of water needed.

Setting a water reservoir up correctly in the first place is really important – it’s easy, but must be done right if it is to work.  We have a handy download here that gives you a step by step guide.  IOTA sells three sizes of round reservoirs suitable for individual plants, and a rectangular reservoir which will serve two or three plants.

In the photos below we show the reservoir sitting on a bed of polystyrene chunks - this could be gravel or any other type of drainage material.

While using a self-watering reservoir in a planter can often be very helpful, they are not appropriate in all situations:

• They are most effective when used with a single fairly large and established plant, or a number of plants of the same size with a reservoir under each (or a long rectangular reservoir). 
• A reservoir must be no more than 10-15cm underneath the rootball to allow capillary action to take place, so if using plants with different size rootballs there will be no way to place a reservoir to serve them all. 
• Annuals/bedding plants will have insufficiently developed roots when they are planted at the start of the season to be able to utilise a reservoir.
• Reservoirs come in different sizes, but even the smallest requires a planter of at least 30cm diameter and 40cm tall