How often and how much should I water my garden? This would have to be the question most clients would ask during a private garden consultation. This is almost impossible to answer without being in the garden of the person who is asking however there are a few basic guidelines that can be used to work out on average how much and how often your plants and garden needs to be watered.
Your soils water holding capability will depend on your soil texture, loam soil is the best as water penetrates and holds well, then there is a sandy soil where the water runs away too quickly and clay soil that is very hard and water tends to run off and not penetrate. Sandy soil and clay soils need a lot of compost added to it then its water holding capability becomes much better.
Soil that is mulched with approximately 7mm of mulch will protect the soil from wind and water erosion and protect it from drying out in the hot sun. Mulching also conserves water as its not lost due to evaporation.
The amount of water your plants will need also depends on a few climate factors specific to each plant as they loose water through their foliage, this could be hot wind exposure, a plant that is in the sun all day, just the morning, the hot afternoon sun or no sun at all. Other factors to consider are competition from near by tree roots, good drainage or poor drainage.
The species of plants in your garden makes a big difference in how much water your garden needs for example some Australian native plants such as Grevilleas survive on rain that comes maybe once a month however an exotic such as a gardenia only survives when the humidity is high and its guaranteed to rain at least once a week.
Plants generally grow there roots close to the surface as this is where the water comes from when it rains, however if you water for longer the water will seep down deeper and this will encourage deeper root growth.The best way to check if your soil is wet enough is to push back the mulch near the drip line of your plant and stick your finger in the soil as deep as you can, pull your finger out and if dirt sticks to your finger and you need to wash your hand that’s good but if you can simply dust the dirt off it’s too dry.
As a guide providing your soil is a loam, mulched, drains well and gets about 6 hours of sun a day between 10 am and 4 pm and is a plant of average water needs such as a Camellia, temperatures are around 21c daily with no rain and winds of up to 15 knots, 10mm of water once a week would be fine. To delivery 10 mm it is the equivalent of 10 liters over 1 square meter. The hotter and windier it is the more you will need and the colder and wetter the less.
To water 10mm with a hose, you will first need to check your water pressure, for example if it takes 30 seconds to fill a 10 liter bucket, it will take 30 seconds to water a square meter of garden bed 10mm. So if you have a garden that is 2 meters wide and 20 meters long you will need to water this bed evenly for 20 minutes.
To water a square meter with a drip system that has drippers 30 centimeters apart, drips 2mm per hour, has four lines 30 cm apart in order to covers the root system of a shrub that is a square meter evenly it would take approximately 27 minutes to water 10mm. Alternativley if you only have one line the 30cm area will require 900 ml which the emiter will release in 27 minutes. To test the rate at which you emiters release water with the water pressure you have place one above a bucket for 1 hour and measure what it collects so you can work out your watering times more accurately.