Though perennial gardening has become a big deal in recent years and many gardeners are no longer keen on installing whole beds devoted to temporary annuals that need to be regularly changed out, annuals still have vital roles to play in the garden. In fact, temporary can be a good thing. It means a greater variety of plants and more options as far as color and style. And if something doesn't work, change it!
Annuals also play well with others, especially perennials and bulbs. They can shine while perennials are still waking up in the spring or dying down in the fall. They make great pairings with bulbs like daffodils while the daffodils are blooming, then hide their fading foliage. Because annuals fill in such seasonal gaps, they're often called 'bridge' plants. So it's smart to always leave space for annuals in your perennial border to keep it looking lively.
Probably the most noteworthy aspect of annuals is color (mostly flowers, but sometimes foliage, as with coleus), which can be used to either echo or contrast with the shades of other plants in the garden. Lilac violas make a sweet monochromatic pairing when planted beside blue grape hyacinths, while hot-pink impatiens with chartreuse hostas or red poppies with blue salvias create vibrant foils for each other.
You can use the same basic planting plan with the same or similar plants every year, or mix it up. If you find something that works really well, it can be a smart idea to repeat it, though the ever-changing array of annual options will likely tempt you to experiment, which is fine too. Keep in mind that perennials and many bulb plantings will increase in size over time, and even shift their locations. You can either adjust your annuals plan to accommodate the changes, or divide the perennials and bulbs and remove portions to keep space for the annuals. And be sure to allow ample room for annuals to achieve their mature heights and widths.
Annuals and their neighbors will get along best if they like the same conditions and care. Obviously the amount of sun and water are critical factors, as is soil type. But many annuals also need a lot of feeding to keep them lush and thriving over their short life spans. So don't plant them with things that need a lean situation.
There are cool- and warm-season annuals, and some annuals have a longer period of interest than others. A few, like zinnias, vinca, gomphrena, and angelonia bloom for as long as or longer than many perennials. For the cool months of the year, use 'hardy annuals' (pansies, violas, ornamental kale and cabbage), which are planted in the fall and last through spring; and 'semi-hardy annuals' (sweet alyssum, wallflower, snapdragons), which are planted in early spring and sometimes again in the fall for brief, jewellike displays.
For the warm months, look to 'tender annuals' such as sunflowers, begonias, calibrachoa, cleome, cosmos, marigolds, petunias, portulaca, scaevola, and torenia, which are planted in spring and last until autumn's first freeze. It's important to know the right time of year to sow seeds and plant the plants based on their hardy, semi-hardy, or tender category. And when annuals are starting to get that past-their-prime look, be ready to replace them. One of the pluses of using annuals is there's always something different to enjoy in the garden.