How to grow perennials in containers

DEAR JESSICA: I’m getting older and only want to use containers. What plants can survive the winter and come back? — Fran Bellows, Wantagh

DEAR FRAN: When shopping for plants that will return year after year, head to the perennials section of the nursery or catalog.

Anything marked for Zone 7 or lower will survive winters on Long Island — but only if you're growing them in the ground or planning to move their containers into an unheated garage or cellar over winter. If you want to leave them outdoors year-round, look for plants with a hardiness rating at least two zones lower, which for us means Zone 5. And even then, after a few overnight frosts it's good insurance to insulate all but evergreens with straw or cover them with frost-protecting crop row fabric until spring. Don't cut back dead plant parts until spring, either, as they'll serve to further protect roots from harsh weather.

Plants growing in containers require more attention than their in-ground counterparts because they depend on you for nutrients (via repeated fertilizer applications) and water (because their root spread is limited).

If you will be planting more than one type of plant in each container, it's important to consider their individual requirements. Combining sun-loving with shade-loving plants, for instance, or those that require a lot of water with those that are drought-resistant, would be a recipe for disaster.

Avoid clay or terra-cotta pots, which tend to crack when left outdoors over winter, and be sure the selected containers have adequate drainage holes in their bottoms. Larger containers, which better insulate roots, afford plants a better chance of survival than smaller ones. Fill them with a high-quality, lightweight potting mix; never garden soil. And don't place pebbles in container bottoms; they may restrict drainage and result in root rot (the only instances in which pebbles or gravel are useful would be when there are no drainage holes in pots). 

I recommend planting one evergreen in the center or rear of each pot to ensure year-round interest, then surround it with shorter flowering plants. You might even add a trailing plant that will grow to cascade over the pot edge. All three will combine for the "thriller, filler and spiller" effect often recommended by designers.

In fall, you can surround the evergreen "thriller" with such "fillers" as annual ornamental cabbage or kale; over winter, you might replace those with artificial poinsettias, if you like. Creeping Jenny, creeping thyme and creeping phlox are colorful perennial trailing plants, or "spillers," but you also could use annuals like licorice plants, trailing snapdragon or sweet potato vine in that role.

Finally, remember that all plants growing in containers, even those that are drought-tolerant, cannot get by on rainwater alone. Be prepared to water them daily during dry, heat spells, and fertilize every three to four weeks.

With the proper care, these plants offer the best odds of year-after-year survival. (Look for varieties marked "compact" or "dwarf.")

Evergreens

Rhododendron "PJM": Dwarf rhododendron grows 3 to 5 feet tall; part sun; lavender-colored flowers in spring. 

Boxwood "Suffruticosa": Dwarf English boxwood grows 2 to 3 feet tall; part- to full-shade; deer resistant, slow growing. 

Ilex "Hachfee," aka "Castle Spire": Narrow blue holly that grows 7 to 10 feet in the ground but can be kept at around 3 feet in containers; part to full sun; the presence of a male pollinator, such as "Castle Wall" will ensure showy red berries over winter.

Yucca "Golden Sword": Grows 3 feet tall with a 6-foot white flower spike in summer; full sun.

Foliage 

Coral bells: Grown mostly for its foliage but blooms in summer with tall spikes of tubelike flowers rising above mounding plants; combines well with golden Japanese forest grass; partial shade.

Hosta: Many varieties and sizes available; combines well with trailing English ivy and creeping Jenny; shade-loving.

Bloomers 

Ajuga: Three-inch tall plants, typically used as ground cover, have variegated, colored or green leaves, with purple or pink flowers in spring/summer; full to part shade.

Bee balm: Dwarf varieties grow to just 15 inches. Attracts hummingbird — not bees. Its name is derived from its use as a remedy to soothe bee stings; full sun.

Black-eyed Susan: Dwarf varieties such as 'Little Goldstar' top out at 10 inches; full sun.

Coneflower: Smaller varieties, such as the new-for-2019 "Double Scoop Bubble Gum" grows 24 to 26 inches tall, sun to part shade.

Astilbe: Feathery flowers, available in many colors, bloom in midsummer, and compact varieties grow to just 12 inches tall; part shade.