Autumn is for planting. Cliche? Yes; but true. Autumn is when most plants are beginning dormancy, and are therefore not so bothered by the discomforts associated with transplant. The weather is cooler and wetter, so that even if they are bothered, such discomforts are not as discomforting as they would be in summer. Once in the ground, plants have a few months to recover before spring.
The two main exceptions to the rule that ‘autumn is for planting’ are plants that are sensitive to frost, and bare root plants. Plants that are sensitive to frost should obviously be planted after the last frost date, at the far end of winter. Bare roots plants do not wait that long, but should wait until they are completely dormant in mid winter before being dug, separated from their soil, and relocated.
Dormant bulbs and bulb like plants, including corms, rhizomes, tubers and tuberous roots, epitomize the autumn planting rule. They must be planted while dormant in autumn or winter. They arrive in nurseries about the same time that they should be planted into the garden. Spring blooming bulbs become available and should be planted earliest. Summer bulbs become available a bit later.
Daffodil, narcissus, tulip, crocus, hyacinth, grape hyacinth, freesia, anemone, ranunculus, montbretia, crocosmia, most lily and some types of iris will all want to get into the ground when the rain starts. Rain leaching through the soil around them, as well as cooler temperatures through winter, tell them what time of year it is, so that they will be ready to bloom when weather warms in spring.
Each type of bulb prefers to be planted at a particular depth. Bearded iris rhizomes want to be buried horizontally, just below the soil surface. As long as the latest get planted within their respective planting season, some types of bulbs can be planted in phases every week or two, so that a later phase starts to bloom as an earlier phase finishes. Daffodil, narcissus and especially grape hyacinth have the potential to naturalize and bloom annually. Montbretia and crocosmia can be downright invasive.