All horticulturists know that the main reason that plants die is improper watering. I see examples of this all the time, both insufficient watering and overwatering, and in this post I’ll explain some ways you can avoid it. But please refer, first of all to the watering guidelines page, which sets out general principles and explains the concept of “establishment.”
The first step is to pick the right plant for your site. If you place a wetland plant in a dry site, you may have to water it pretty much forever (although some wetland plants do quite well in dry sites, once properly established). A bigger problem is the opposite one: placing a plant adapted to dry conditions in a wet site, or in a site that’s watered continually. The plant is quite likely to rot away.
Many people start gardening by planting annuals, and when they go on to perennials, grasses, and shrubs, treat them like annuals. Annuals are plants that live for a single season. They include all vegetables and many ornamentals such as impatiens and marigolds. Because of their short lifespans, they do not develop large, deep root systems, so they do need continual shallow watering. (This is a big reason that I discourage clients from using annuals as ornamentals–growing them is just not sustainable.)
Most ornamentals, certainly almost all native ornamentals, are perennials: their lifespan ranges from several seasons to pretty much forever. These plants develop large, deep root systems. They put down roots that can mine the soil over a large area for minerals and moisture. To encourage them to develop these deep root systems, water infrequently, if at all, once they are established. A little drooping on a hot afternoon will not harm them. Recent research shows that stress is good for plants: it encourages them to grow deep, strong roots.
What does this mean for the average gardener? It means the following key points:
— during establishment, perennial plants need supplemental watering during dry periods only.
— once established, properly sited perennial plants do not need supplemental water except perhaps in periods of extreme drought.
— the rule of thumb for newly planted trees is one year of supplemental watering per inch of trunk diameter. So, for example, a tree with a 2-inch diameter should receive supplemental watering during dry periods for two years.
— established lawn grasses, which are perennial, do not need supplemental water, and if treated this way, they will go dormant during hot, dry periods but green up again with cooler weather or rain
— if you cannot tolerate a dormant lawn, provide deep but infrequent watering to encourage the development of deep roots
— for annuals such as vegetables, deep watering once or twice a week is better than daily shallow watering, which can lead to rot and all kinds of fungus diseases