You already know rainwater is good for your garden and may have noticed how green your plants look after a good rainfall. That’s because rainwater contains nitrogen which “washes out” of the atmosphere, providing your plants with a nutrient boost, giving them their green colour. So it makes sense to catch and store this free resource to irrigate and feed your garden, and one of the best ways to do that is a low-tech do-it-yourself method known as a contour swale.