All you could ever want to know about irrigation...

Plants in pots rely on us almost entirely to supply their needs- being artificially separated from natural soil means that they are unable to send their roots great distances in their quest for food and nourishment. Whether a plant is potted in a garden or a nursery it must be cared for with the same diligence that a person uses to ensure the well-being of an animal. Because plants cannot 'tell' us they are hungry or thirsty means that we must always be looking for the signs, and that we must be ready to act at very short notice.

Watering large numbers of plants by hand is very time consuming, so irrigation systems are installed to save time. Here is an explanation of the methods of irrigation that you will see on most nurseries, either as an employee or a visitor....

On the nursery we have lots of separate areas irrigated by a complex system of pipes and valves controlled by computers (see left); if an area is uniformly dry we assess it's requirements and, if necessary, run the irrigation for an appropriate length of time. Likewise if an area is expected to go dry (e.g. on a dry weekend) we can assess the area's requirements and set up the automatic system to water when we're not around. On a busy nursery the ability to water large areas overnight saves a lot of time and allows staff free access to tunnels etc. during the day without getting wet! Also being able to water areas during the night allows the irrigation cycle and hand watering activities to be spaced out sensibly over 24 hours, spreading water consumption out through the day, which in turn means that we don't need as much storage- this is crucial if you have no mains water!

All irrigation systems use broadly the same principle: run water through a section of the irrigation system (either sprinkles, drip lines or flood beds) for a period of time which has been calculated to allow each pot to receive the necessary amount of water. Let's take, for example, a drip system which delivers 1L of water in 10 minutes; if you need to deliver 10L of water into your pots to water them properly you will need to run the system for 100 minutes to make sure they are watered.

Most of the irrigation on the nursery is done by a network of overhead sprinklers which spread water over the plants from above. These are easy to install and run but are fairly wasteful of water, watering over access routes etc., and wet foliage is often a contributing factor with leaf diseases, where the drop of water on the leaf surface acts as the perfect environment for spores etc. to get growing.

The overhead sprinklers in action!

Overhead sprinklers in a tunnel- basically the same as outside but upside-down!

The more efficient watering technique is a drip line system. Here small tubes deliver water from a header pipe directly onto the surface of the compost, usually with a spike to hold the pipe in place, stop it getting blocked up (which it would do if the pipe was pushed straight into the compost) and guide the water to where it is needed. The advantage of drip irrigation in containers is that the water is delivered to exactly the place where it is needed at the roots, rather than onto leaves, access roads, straight into drains or onto staff! Also by watering straight into the compost the surface of the compost is kept drier, and this makes it harder for weed seeds to germinate. However, these systems are fairly expensive to install but work wonders for larger stock and any stock tied to lines, but on growing areas where plants are smaller and close together this system would just too complicated and unmanageable to be useful.

A typical drip spike...

Flood beds are a different way to irrigate; stock is growing in what is essentially a large watertight tray with a tap at one end and a drain at the other. To irrigate you simply close the drain, open the tap and fill the area to the required depth with water, leave the plants to soak the water up and then drain the area. The advantage of this system is that you can water a lot of plants from below, directly into the compost, without wetting the leaves. Staff can also work around the area without getting wet (unless they fall into the beds!). There are disadvantages though; these beds are fairly costly to put in, must be pretty well flat, and grow slippery algae on the floor of the bed and on the pots (which need to be cleaned before they are presentable for sale). In order to be efficient with water use the water drained from the flood bed must be collected, stored and cleaned to remove unwanted fertiliser or chemicals before being reused- for most nurseries this would be a horrendous expense for little gain.

Note the algae on the tunnel plastic- stray irrigation water can cause problems.

 Here's a table summarising the benefits and drawbacks of different irrigation systems when compared with watering by hand.

	Overhead	Drip lines	Flood Bed	Hand Watering
Easy set-up	yes	no	no	yes
Cheap	yes	no	no	mainly labour
Water Efficient*	no	yes	debatable	yes
Suitable For Feeding	no	yes	no	yes
Run-off problems?**	yes	mostly no	algae problems	mostly no
Automatic Control	yes	yes	yes	no

*   do not waste much water.

** excessive run-off can cause problems with mud and/or algae.