The reason is simple. The plants are usually shallow rooted and have had only a matter of weeks to establish themselves. They cannot draw on the reservoir of water deep in the soil, and the plants most at risk are the newly planted ones and those grown in containers. With these situations you must water regularly and not just during prolonged dry weather.
The battle against water shortage should begin well before the dry days of midsummer. Incorporate adequate organic matter into the soil before planting, the compost used in containers is already ideal in this respect. Make sure that the soil or compost is watered well at planting time and mulching gardening beds is another way to retain moisture.
Soil with an average cover of plants loses about two gallons of water per square yard each week in summer. This is equivalent to half an inch of rain or applied water. If there is no rain and you have not watered then this water must come from the soil or compost reserve and drying out occurs. If water is not applied then the plants start to suffer, to avoid problems we will follow these simple rules.
Rule One: Don't wait until the plants have started to wilt. The time to begin watering is when the soil is dry to a depth of two inches or more and the foliage has a dullish tinge. When this occurs it depends on the weather and soil type. For example a sandy soil dries out much more rapidly than a heavy one, and low-humus soils hold less water than organic-rich earth. My soil is sandy therefore I am careful and monitor the plants progress on a daily basis.
Rule Two: Once you decide to water then do the job thoroughly. Applying a small amount ever few days may well do more harm than good as it encourages surface rooting and weed seed germination.
Rule Three: Decide on the watering technique. Point watering is the simpler method; this calls for holding the spout of a watering can close to the base of each plant and then adding water gently to soak the ground around the root zone. Troughs, small beds, pots, window boxes and hanging baskets can be tackled in this way, but it is extremely time consuming if the area to be covered is extensive as you may need to apply two to three gallons per square yard. For large beds and borders with many plants of various sizes the obvious choice is overall watering. This involves applying water to the whole area rather than treating each plant. You can try watering over the top of them with a watering can which is filled from a hosepipe but the usual procedure is to walk slowly along the border or around beds with a hand-held hose and a suitable nozzle. This may seem to be boring but I truly enjoy watering. This gives me an opportunity to check the progress of each plant and keep a close eye for any sign of trouble. A common error is to move too quickly so that the plants receive too little water. A sprinkler which can be moved along the border at regular intervals is usually more satisfactory. Always carry out this task in the evening or early morning. Do not apply the water to the grass or gardens in the hot sunshine as this has a tendency to burn the roots. Trickled irrigation through a perforated hose laid close to the plants is one of the best methods of watering.
Rule Four: Repeat the watering if rain does not fall but do not try to keep the land constantly soaked. There must be a period of drying-out between waterings. As a general rule you will need to water every seven days if rain does not fall. The precise interval depends on many factors. To check if water is needed the best plan is to examine the soil two inches below the surface. Water immediately it is dry.
Rule Five: When planting new specimens make certain you water the plants every day for the first two weeks. The only days you need not bother are when it rains. This is a mandatory rule I follow faithfully. Be it tree, shrub, perennial or annual, when that plant arrives on my land it is watered for fourteen days after planting. An expert nurseryman prescribed this advice some years ago and I have followed his guideline with overwhelming success.
It has been stressed that small containers will need frequent watering in hot and dry weather. This is a time-consuming but not difficult task in most cases. It can be a problem with overhead containers such as hanging baskets and window boxes. It is possible to use steps and a watering can in some instances, but there are three alternatives which make the task easier.