One of the biggest challenges with growing plants in pots or containers is irrigation. It is very easy to over irrigate or under irrigate a plant in a pot. Over irrigating results in the leaching of nutrients from the soil and this can result in weak, spindly plants. Under irrigation is something I need not describe, as I’m sure we’ve all come home after a vacation to find a withered plant in a pot.

Here are a few insights that may help keep your potted plants well watered.

The Bigger the Better

A large pot is easier to manage because it can hold a lot of water and nutrients in reserve in the soil. If you are shopping for a pot to grow a plant in, get the biggest one you can afford or accommodate in the garden. It will make your life easier, as long as you don’t have to move it around a lot.

How Much Water

The best way to check the soil for water is to stick you finger in it. With a pot, pick it up and see how heavy it is. If it’s in need of water it will be light. Apply enough water to thoroughly wet the root zone of the plant and recharge the soil with water. Once water starts leaching out the bottom it’s had enough.  With established plants, allow the surface of the soil to dry out between waterings. This will help reduce disease problems.

The weather and the stage of the plant will also have significant effects on water requirements.  If the weather is hot, dry, and windy more watering will be required.  A large vining  tomato plant in a bucket can suck up a gallon or more of water on a hot day, and it is during these times that growing anything in a pot is a real challenge.

Hydrophobic Soils

If water is pouring out of the bottom of the pot after only a couple of cups of water the soil may be hydrophobic and repelling the water.  When soil dries out it can form a layer at the surface which acts like a thin layer of wax and prevents the soil from absorbing the water.  Roughing up the soil around the plant and poking some holes into the soil with a pencil or screwdriver will solve the problem. Adding a drop of dish soap to a litre of water will also help the water penetrate a dry soil, but I find it unnecessary.

Drainage

Make sure your pot or container allows for drainage of excess water. This is particularly important if the pot is outside because the rain will fill it and make it a pool. No need to put stones or gravel on the bottom of the pot, just ensure there are holes on the bottom. Stones on the bottom of a pot are not necessary and can hamper drainage.  Good drainage is essential for long term health of a plant. Without good drainage salts can build up to toxic levels over time.

Root Bound

If the entire pot appears to be full of roots, your plant is probably root bound. It is difficult to water a root bound plant because the soil is so full of roots there is no where for the soil to store water. If it is root bound, then the general procedure is to transplant it into a bigger pot. Mint, which is extremely invasive and a good candidate for a pot, often becomes root bound. The best strategy with mint is to remove a few healthy clumps of the mint rhizomes and plant them into a fresh pot each year. With mature annuals like tomatoes, transplanting may prove too difficult or detrimental to the plant, and so plan for a bigger pot next year.

Irrigation

Automatic irrigation is a great way to deal with your watering woes. An automatic water timer attached to some tubing and emitters and you’re off to the races. A pressure regulator and backflow preventer should also be installed. There are hundreds of different spray nozzles or emitter, but for pots I use a simple dripper that drippers out about 4 litres an hour. For long window sill boxes you can use a small diameter soaker hose which weeps water all along its length or a drip pipe which has drip emitters every foot. I often hook up a system to water my tomatoes when I’m on holidays. It waters each tomato plant with a dripper. The system consists of a water timer, pressure regulator, backflow preventer, some tubing, connectors and emitters. Most of this equipment is widely available at the big hardware supplies stores. Sometimes it is referred to as micro-irrigation to differentiate it from the larger landscape irrigation systems. Lee Valley has excellent information and selection of irrigation supplies, and many local irrigation supply stores also offer a good selection of parts.

Alternative Methods

There are a variety of alternative methods to watering, such as self watering pots and water gel crystals.  The self watering pots come in a wide variety of shapes and sizes and the internet offers countless ways to make your own. Self watering pots supply the water from the bottom where the soil and roots wick it up. They are best for indoor or patio use where they are protected from the rain. Salt build up in the soil can become an issue over time if the soil is not routinely flushed, and the stagnant water can foster insects if the design is poor. 

The water gel crystals are polymers that retain water and release it as the soil dries out.  Hanging baskets are a common place where water gel crystals are used, and they can reduce the frequency of watering significantly. Simply add a tiny amount of the crystals to the lower portion of soil in the pot when potting up. Follow the directions and do not over apply or the pot will overflow when the crystals expand with water.