How much water do your plants need? It depends on your soil and climate, as well as on the particular plants and how you water them, but the answer is often less water than you think.
Plants Have Preferences
If you choose plants that are naturally adapted to your landscape, they will have a better chance at settling in successfully, and they should need little help from you after they are established. The key is to match plants' needs with your conditions, and to help them get established.
One sign that they are thriving is their physical health. A sign that they are well-adapted is their ability or propensity to reproduce successfully in your landscape.
Soil and Topography
There are plants that grow well in nearly every soil and climate. Some prefer clay and can live happily with soil that is waterlogged, parched to cracking, and frozen by turns. Others prefer quick-draining sand dunes, still others a deep, moist, humusy forest loam.
Every soil's composition (parts of sand, clay, loam, and humus) affects how much water it captures and how long it holds that water. The contours of the ground also influence how quickly water flows through an area. Lower spots will catch more runoff, while higher ones will shed it more quickly.
Plant Size and Origin
The plants' sizes and their previous care play a part in their adaptability. Larger plants generally need more time and care to adjust to transplanting than do small seedlings. Nowadays I buy most plants as small as possible (6-packs of perennials and liners of trees and shrubs); they cost less and require less attention than the bigger ones.
I also avoid buying from nurseries that pamper their plants with fertilizers and frequent shallow waterings. Those plants are going to expect pampering in my garden too, and when they don't get it, their health will likely suffer.
Plants' origins on a larger scale matter too. Plants that are native to your landscape have evolved to survive (even thrive) in your region's climate, and except in very unusual weather, they will be able to handle the amount and frequency of precipitation they experience in your landscape.
Plants' Response to Drought
Plants that are drought-tolerant by nature can look great when they're receiving no water. Their foliage can be lush and full.
Other plants, often the larger-leaved ones, may wilt to conserve water by cutting down on their transpiration, but this is normal behavior for them, and they'll fluff out again as conditions become moister. Some may even go dormant and turn brown (many cool-season grasses naturally do this in the heat of summer), but they are not dead. Adequate rain will bring them out of dormancy.
Some plants may need moister conditions than your location's climate provides. You will likely always have to provide these with supplemental water, unless you can cleverly site them in a moister microclimate (in a basin or mulched deeply with leaf mold or protected by taller plants all around, for instance).
You've probably heard the adage: "water deeply and infrequently." This really does create healthier, less dependent plants (and therefore less work for the gardener). In an area where water comes often, plants focus on top growth rather than developing deep roots. A period of drought (perhaps caused by your two-week holiday) will stress them and may cause them to abandon some of that water-hogging foliage.
Plants that are used to periods of days or weeks without water will develop larger root spreads (and will consequently hold soil better, useful when you are gardening on a slope or other erosion-prone site). They will be better able to maintain the foliage they've grown, and will not wilt at the first sign of neglect.
Unless the weather is overcast and rainy, I water newly planted plants daily for two or three days after planting, then every other day for the next week or so. Depending on the weather, I will continue to water every few days to every week for the next several weeks. After that, we usually receive enough rain here that additional watering is unnecessary except during prolonged periods of drought during the plant's first year in my garden.
Since I plant species adapted to my climate, I expect most plants to survive the occasional drought without supplemental water after that first year. If I really like a particular plant, I might soak it a few times during the second year, but by the third year, if it can't survive, I give up and give it away. (I don't water any of my plants routinely; that's one way to keep maintenance low...)
I do mulch most new plants to help them retain moisture, unless they prefer dry or well-draining conditions. Some like wood chip mulch, some (woodland and wetland plants that don't like to dry out) prefer old leaves, which I pile up to a foot deep and then plant into. Others, like herbs and plants that want quick-draining soil, do best mulched with gravel, which keeps the ground surface dry and warm. I give them as much mulch as I can afford—usually two to four inches deep and extending out to their drip line.
I have followed this regime in two different gardens, one urban with over six inches of loam topsoil and the other rural with dry to mucky clay in place of topsoil. The plants that survive their first year generally grow to robust health (unless they are eaten).
Retraining Pampered Plants
It's all very well to start plants on a strictly limited water diet, but it's another thing to switch plants to that diet if they're used to frequent, shallow watering. Such a change in conditions must be undertaken gradually and may cause variable reactions. Some of your plants may adapt easily to less watering, others may require a season, and a few might go on needing supplemental water during droughts for the next five to ten years.
The best thing you can do is wean them gradually. Begin right away to water deeply instead of shallowly. The top one to one-and-a-half inches of soil should generally be damp after you water. You might save a lot of water if you have the time to direct a stream of water onto the crown of each plant for a minute or so (and up to ten or twenty minutes for shrubs and larger trees) rather than sprinkling the whole area, but this individual watering approach will take more time. I try to look at it as an opportunity to commune with my plants, getting to know them and also letting them know I care.
At first, you may want to check them every day or two to see how long the soil stays damp. Poke your finger into the soil, and if it is dry an inch down, water deeply again.
Gradually let them get accustomed to periods of dryness between waterings. There will be a limit to the dryness they can stand (they may be descended from swamp lovers or desert dwellers; this is where knowing their genetic origin comes in handy), but some of them may surprise you, and all of them will be healthier if they're encouraged to develop fuller root systems.
As the psychology books might say, we create our own reality. I require fairly quick independence from my garden plants, and I don't give the dependent ones extra care, so I am left with plants that succeed with a minimum of intervention.
Similarly, your watering schedule will influence which plants succeed in your garden. Your garden adapts to you. Therefore, I think it is safe to say that your plants need exactly the amount of water you give them.
Thomas Christopher meanders around the US interviewing waterwise gardeners and describing their techniques and their gardens.
For a visually and verbally enticing introduction to dryland plants, try Lauren Springer's The Undaunted Garden: Planting for Weather-Resilient Beauty.
Lauren Springer married fellow garden author Scott Ogden, and together they wrote an even better book for dryland planting, Plant-Driven Design: Creating Gardens That Honor Plants, Place, and Spirit.
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