Water, Water, Water
Water is the key to everything in the garden. In fact, no less an authority than Gerould Wilhelm of the Morton Arboretum counsels that good garden design begins with an understanding of the natural water cycle of the particular site. Its proper management and conservation will always lead to a beautiful, trouble-free landscape.
In any case, new plantings need abundant watering, even in the best planned gardens. The number one reason plants fail to thrive in the new landscape is drastic under-watering by super busy, well-meaning customers who simply don't understand the quantity and frequency of waterings needed to properly establish new plants. Often temporary sprinklers, which are useful for establishing turf, do not water deeply enough to be effective on trees, shrubs and perennials. Hand watering with a hose is much preferred.
This urgency is doubled at the end of the season, when chillier temps and increased rainfall prompt homeowners to water even less, wrongly assuming the critical time has passed with the summer heat. The result is often disastrous for any species planted after September 15th or so.
When installed late in the season, plants may not show much growth on top, but underneath they are striving to lay down some firm new roots before winter. And they need water to do this. Root growth in Fall might be minimal, but every little bit is crucial to overwintering specimens. Further, plants do better when their roots freeze moist, not dry.
Leave your hoses at the ready until the weather forces you to take them indoors. (In today's climate, this is often nearly Christmastime) Autumn nights may drop temperatures below freezing, so be vigilant at the spigot, but leave your hoses available to water deeply any new plantings you have.