Once your landscape planting has been completed, proper watering becomes the most important part of plant survival and appearance. Transplanting causes a certain amount of shock to a plant due to the loss of roots, transportation and handling, and a change in growing conditions.
In order to help plants through their critical first year, it is important to understand a little about soil in your area. While some arid regions contain sandy soils that drain very quickly, other soil is made up of a high percentage of clay (finer soil particles). Clay soil tends to drain very slowly, and this is especially true in new developments where bulldozers and heavy equipment have compacted that soil.
When you consider the combination of clay soil and the ample rainfall many areas in the northeastern US receive, it seems as though watering shouldn't be necessary. While this is usually true of well-established plants, transplants are very dependent on regular watering. Plants grown in plastic containers have a fast-draining soil mix, and are put under additional stress when circling roots are sliced at planting time. Also, balled and burlapped (B&B) trees lose a large portion of their roots when they are dug at the nursery.
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So what is the best rule of thumb for watering new plants? Water plants thoroughly when natural rainfall is less than one inch per week. To check the weekly amount of rainfall, you can purchase a rain gauge or use something as basic as a coffee can.
When watering becomes necessary, plants should be thoroughly soaked once a week. Apply water slowly and repeatedly to allow time for it to soak into the ground. If the weather has been dry, and you are watering once a week, it is usually difficult to give plants too much water.
Occasionally you'll plant trees in very hard compacted clay, usually in new neighborhoods or townhouse developments. In this case, we recommend that watering intervals be extended to once every two weeks for large trees, since these planting holes can act like "bathtubs" the way they hold water for long periods of time.
Once a new lawn has been seeded, watering becomes the most important part of the job. Watering is so important that it can make the difference between luxuriant success and total failure. Failures further delay the look of a beautiful lawn and translate into even more watering. So the key is doing it right the first time.
The most important thing to remember about grass seed is that it must be in contact with the soil surface to germinate (sprout) properly. Successful germination also depends on ample moisture at the soil surface, and soil can dry out very quickly on sunny or windy days.
Therefore, during the first few weeks of watering a new lawn, you need to concentrate on keeping the lawn surface moist. This is best accomplished by frequent light watering, especially on hot, sunny days. Areas of the lawn in full sun need watered more often than partially shaded areas. Lawn edges and curb areas are easy to miss, and sloped areas always dry out faster than level ones. The best rule of thumb is: IF IN DOUBT, WATER!
Morning is best, even if it is just before sunrise. Afternoon watering is fine too (even required some days) but more water is lost to evaporation. Late evening is the time to avoid watering a lawn if possible. Why? Turfgrass watered in the evening tends to stay wet all night, providing an ideal environment for fungus diseases. Young seedlings are especially vulnerable to certain fungi which can wipe out large areas of turfgrass overnight! Particular caution with evening watering must be exercised during hot, humid weather, i.e. summer nights when it is muggy.
Watering newly sodded lawns is nearly identical to the watering instructions shown above for seeded lawns but the frequency and duration should be slightly less. In other words, after giving the sod a thorough watering initially, you can probably water it about half as often, for half as long, as you would water a seeded lawn.
My old turfgrass professor also recommended using a lawn roller to roll new sod after three to five days to remove air pockets and further aid with good "sod to soil" contact.