When gardeners and landscape designers speak of "sequence of bloom," they are referring to the different time periods in which various landscaping plants bloom during the course of a growing season. Once privy to this knowledge, you can plan your landscape accordingly, so as to have something in flower at all times during the months conducive to flower gardening in your region. Guided by such information, plant selection at the nursery becomes more purposeful, rather than being based on whim ("Oh, that looks pretty, let's buy it").
Information on sequence of bloom is sometimes organized in charts. For example, such a chart could have the names of plants running down the left-hand side of the page (alphabetically), with month names running across the page at the top. Horizontal and vertical lines are then drawn accordingly to divvy the page up into cells. The cells can then further be broken down to indicate weeks. If the plant in question blooms for two weeks in May, let's say, check marks are placed in the corresponding cells.
The nice thing about these sequence of bloom charts is that the order will always be the same, regardless of region or freaky weather. For example, if someone in Maryland (U.S.) composes such a chart, and I (a Massachusetts gardener) compare the results to my own, I'll see that snowdrops, say, bloom later for me (because I'm further north), but they'll still come into flower at about the same number of weeks before garden phlox as they do for the Maryland gardener. Of course, there are some plants that I cannot grow in my planting zone that a gardener in Maryland can grow, but that's a separate issue from sequence of bloom.
You can compose your own sequence of bloom chart, but I think it makes more sense for the gardening newbie to begin with a simple garden journal. That is, when the first spring flowers come into bloom, note this fact in your journal (their names and when they bloomed). Thereafter, as the growing season progresses, continue to jot down such information. In addition, indicate when each plant stops blooming.
As a newbie, the record in your journal's first year may well be pretty sparse, with numerous gaps (i.e., weeks during which nothing was in bloom). That's OK: it's all part of the process. The idea going forward will be not only to acquire more plants, but specifically to acquire specimens whose blooming period will allow you to plug those gaps. Beyond that, your plant choices will be just a matter of taste. Gardeners may mix and match in pursuit of continuous sequence of bloom from among various groups of flowering plants:
You'll find that growing long-blooming perennials makes your sequence-of-bloom goal easier to attain, because the longer your selections stay in bloom, the less you have to worry about plugging gaps. For the same reason, annuals come in particularly handy, since they will generally bloom for months on end if you deadhead them. Annuals will still be supplying floral color at the tail end of summer and right into fall -- until the first frost -- weeks or even months after many plants have long since packed it in for another year.
To help you with selecting shrubs in the pursuit of sequence of bloom, I have composed the following lists:
Are you sold on the idea of planning your plant purchases in this manner? If so, you now understand not only what "sequence of bloom" means, but also how important the concept is in designing a colorful landscape.
So What Does "Successional Interest" Mean?
Some garden writers use "sequence of bloom" and "successional interest" as if they were synonymous. I do not, and I'll explain why now.
I use "sequence of bloom" in the narrow (literal) sense, whereby it can pertain only to specimens with blossoms. By contrast, I reserve the terminology "successional interest" to refer to a broader range of plant characteristics than just blossoms that you can use to keep your landscaping colorful as consistently as possible. In other words, in my lexicon, sequence of bloom is a subset of successional interest.
Here's an example where I would speak of "successional interest" rather than "sequence of bloom." Even after the frosts of early fall have killed flowers in the North, one can enjoy the color offered by fall foliage trees and shrubs with good fall color (assuming you've had the foresight to install such plants).
Must one give up the pursuit of successional interest after the autumn leaves drop to the ground? No, not if you've planted evergreens and other plants with winter interest, as I discuss in my article on winter landscape ideas. Even during seasons when there are flowers to enjoy, you can supplement the visual interest they furnish through the use of foliage plants. More generally speaking, you should always consider the forms and textures of plants when planning your landscape design if your goal is year-round interest in the yard.