Tender Perennial Plants: Care Of Tender Perennials In Gardens
By: Jackie Carroll
Native to warm climates, tender perennials add lush texture and tropical atmosphere to the garden, but unless you live in warm climate zones, winter can spell disaster for these frost-sensitive plants. Keep reading to find out more about tender perennials.
What Are Tender Perennials?
Tender perennial plants come from warm climates where they don’t need the ability to withstand cold winter temperatures. When we plant them in cooler climates, they won’t survive the winter without special care.
Some tender perennials such as begonias, calla lilies, and caladiums add lush foliage or fantastic flowers to shady spots. Many of these shade-loving tender perennial plants come from tropical rainforests where they are protected and shaded year round by the rainforest canopy. These plants need soil that is rich in organic matter and plenty of water.
Other tender perennials come from warm, Mediterranean climates. This group includes tender herbs such as rosemary and cilantro, as well as fragrant shrubs like bay laurel. These plants generally prefer soil that drains freely and lots of sun.
Care of Tender Perennials
Plant tender perennials in the garden in spring when there is no longer a danger of frost. Keep the soil moist until they become established and then water and fertilize according to each plant’s needs. Tropical plants usually need weekly or biweekly watering in the absence of rain. Mediterranean plants don’t usually like much fertilizer, but other tender perennials like a light dose of fertilizer in spring and midsummer. Prune them as necessary to keep the plant looking neat and encourage new growth.
In the fall, gardeners in temperate climates face a dilemma. The easy solution is to grow them as annuals, replanting every spring. While this may be the best way to go for inexpensive plants and bulbs, you may want to save some of your more expensive plants and those with sentimental value.
The limiting factor is finding a place to store your plant material. Root cellars are ideal, but since most people don’t have one, you’ll have to find a dry location where you can maintain a temperature between 50 and 55 F. (10-12 C.) all winter. A spare room where you can close off heating vents or a cool garage works well if you can keep the temperature from dropping too low.
After the foliage on bulbs, tubers and corms dies back, dig them up, trim off remaining stalks and stems, and lay them out in a single layer to cure at room temperature for a few days. When they are dry, brush off the remaining soil and store them in open boxes filled with sand, peat moss, or vermiculite.
Plants that don’t grow from bulbous structures can overwinter indoors as potted plants, or you can take cuttings in late summer to start over winter. Cuttings don’t take up nearly as much space as full grown potted plants, and they usually grow better when transplanted outdoors in spring. If you want to use a tender perennial as a houseplant over winter, cut it back by about half before potting it up.
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Planting Tender Perennials in the Garden
Tender perennials are attractive additions to the home landscape. Since they are not reliably winter hardy in Iowa, tender perennials must be planted outdoors each spring. Examples of tender perennials include gladioli, dahlias, cannas, tuberous begonias, caladiums, and calla lilies. Tips for planting the aforementioned tender perennials are provided below.
Gladioli Glads should be planted after the danger of frost is past, about mid-May in central Iowa. (Gardeners in southern Iowa can plant about 1 week earlier. Plant 1 week later in northern areas of the state.) Make successive plantings every 2 weeks for continuous bloom throughout the summer. The final planting should be made in early July. When purchasing glads, select bulbs (corms) that are 1½ inches or larger in diameter. Large-sized corms will produce attractive flower spikes. Corms smaller than 1½ inches in diameter will produce foliage, but may not bloom. Planting depth varies with the size of the corms. Large corms should be planted 4 to 6 inches deep and 6 to 8 inches apart. Small corms should be planted at a depth of 3 inches. Gladioli require well-drained soils and perform best in a sunny location. Glads typically bloom about 8 to 10 weeks after planting.
Gladioli require little special care during the growing season. Control weeds by frequent shallow cultivation or by mulching. Water weekly during hot, dry weather. A 10-10-10 fertilizer may be applied as a sidedressing about a month after planting. Staking will be required in windy, exposed areas.
Dahlias Dahlias perform best in a well-drained soil in full sun. (Plants that require full sun should receive at least 6 hours of direct sun each day.) Dahlias often rot in wet, poorly drained soils. Tuberous roots radiate out from the dahlia crown like the spokes of a wagon wheel. Viable tubers must have an eye originating from the crown portion in the center, plus a neck that connects the crown to the body of the tuberous root. Carefully divide the dahlia crown with a shape knife, scissors, or pruning shears.
Before planting tall cultivars, drive a sturdy support such as a metal fence post or wooden stake into the ground. Do this prior to planting to ensure that the tubers are not damaged by the post or stake. To plant tubers, dig holes about 6 to 8 inches deep. Place the tuber horizontally in the ground. Then place soil back in the hole. The dahlias should emerge in about 2 weeks. To avoid frost or freeze damage, plant dahlias in mid-May in central Iowa.
Cannas Canna rhizomes can be planted outdoors in mid-May in central Iowa. The rhizomes should be planted 4 to 5 inches deep.Cannas perform best in moist, well-drained soils in full sun. During dry weather, water plants once a week. To promote growth, apply a balanced garden fertilizer, such as 10-10-10, to the area prior to planting and once or twice during the growing season. Remove spent flowers to maintain their attractive appearance and promote additional blooms. While cannas have a few insect and disease pests, none are considered serious.
Tuberous Begonias Plant tuberous begonias outdoors after the danger of frost is past. Tuberous begonias perform best in moist, well-drained soils in partial shade. (Partially shaded sites typically receive 2 to 4 hours of direct sun each day.) In the home landscape, areas that receive morning sun and afternoon shade are often excellent planting sites. The planting site should also be sheltered from strong winds. Use a well-drained potting mix when planting tuberous begonias in pots, window boxes, and other containers. When planting tuberous begonias, place the plants at the same depth they grew in the container. Planting them deeper than previously grown may encourage the buried portion of the stem to rot.
Once planted, watering, fertilizing, and deadheading are the primary maintenance chores. Tuberous begonias should be watered weekly in beds and borders in dry weather. Plants in containers will have to be watered more often. Check container-grown plants regularly (daily or every other day) and water as needed. In garden areas, apply an all-purpose garden fertilizer, such as 10-10-10, to the area before planting. Fertilize container-grown tuberous begonias every 2 to 4 weeks with a dilute fertilizer solution. Remove spent flowers to improve their appearance and encourage additional blooms.
Caladiums Caladiums can be grown in beds, borders, and containers. Plants perform best in moist, well-drained soils in partial shade. Caladiums can be planted outdoors in mid-May in central Iowa. Use a well-drained potting mix when planting caladiums in containers. Plants should be placed at the same depth as previously grown.
While caladiums enjoy warm weather, they don't tolerate dry conditions. In beds and borders, water plants on a weekly basis during dry weather. To help conserve soil moisture, apply a 2-inch-layer of mulch around the plants. Caladiums in containers should be checked regularly and watered when the potting soil begins to dry out. Caladiums have a moderate fertility requirement. In beds and borders, apply an all-purpose garden fertilizer and incorporate it into the soil before planting. Fertilize caladiums in pots with a soluble fertilizer every 2 to 4 weeks.
Calla Lilies Calla lilies should be planted outdoors after the danger of frost is past. Calla lilies perform best in moist, well-drained soils in part sun (4 to 6 hours of sun) to part shade (2 to 4 hours of sun). Before planting, apply an all-purpose garden fertilizer to the area. Calla lilies like a consistent supply of moisture during the growing season. Water weekly during dry weather.
(Foxglove-Tender Perennial) We are so excited to bring to you DIGIPLEXIS® ‘Berry Canary’, a long blooming flower much like that of Foxglove. Standing 24 inches tall and 20 inches wide, DIGIPLEXIS® ‘Berry Canary’ is a prolific bloomer. The flower spikes contain hot pink flowers with creme throats speckled with dark wine dots that line the whole stem. ‘Berry Canary’ has a tighter, more compact habit than that of other Foxglove giving it a tidier look. DIGIPLEXIS® are hardy in zones 8 and higher, although the longevity of the blooms makes this tender perennial a must have for every climate gardener. This easy to grow, deer resistant variety grows best in full sun to part shade locations. Enjoy DIGIPLEXIS® ‘Berry Canary’ in container designs as well as in your perennial beds.
For general growing tips and how to care for your perennials. View Garden Crossings How to care for Perennials page.
Interested in growing tips for other plants we sell. Browse our Plant Care Tips.
Dividing Tender Perennials
As part of your routine tender perennial care in fall, you may also want to take the opportunity to divide plants that are looking a little disheveled. Older perennials tend to exhibit a “doughnut effect” where the plant tissue over the centre of the root ball is looking a little sad while the tissue around it is flourishing. This is a good sign that the plant could use splitting up.
The best time to divide perennials is right now, when the evening is noticeably chilly and the soil is warmer than the air. Start by digging around the drip line—that’s the perimeter of the plant where water would land when it drips off the leaves—and dig out the root ball carefully. Dig around the roots curving inward, as if you’ll be leaving a cereal-bowl shaped hole in the ground.
Once you get the root ball out, examine the roots. It should be pretty obvious which roots look healthy and which don’t. Remove the rotten stuff—don’t worry, the plant is dormant by now—and with a sharp, disinfected knife, cut through the crown to divide the root ball in half without slicing all the way through it. Then, gently separate the two halves. The idea is to only cut where you have to and try to untangle the rest as well as you can. If you find yourself pulling roots apart, use the knife to cut them.
Now, do it again with both sides. Ideally your single root ball should end up as four equal-sized root balls. Plant them immediately in holes enriched with compost. If the soil is clay-heavy, make the hole a little wider and mix in some peat moss.
And that’s really about it! Tender perennial care is really nothing to stress about, it just involves taking a little time to baby your more delicate garden residents. If you’re in the Edmonton area, you can pick up all these supplies at our garden centre in Sherwood Park. Hope to see you before the snow!
The propagation method for each perennial is listed in Table 1. Division is the quickest and easiest method of multiplying some clumping perennials. Simply dig the plants and shake off the soil. It will be apparent where to separate the plants into smaller units having roots and leaves. The best time to divide plants is after the blooming season or during milder times of the year. Perennials that are frozen back to the ground can be lifted, divided, and reset at that time with good results. Some perennials are easily grown from seed or cuttings as well. Spring and summer are the appropriate seasons for these forms of propagation.
Tender Perennials Need Indoor Protection
Dig Elephant Ear before frost.
Photos by Rosie Lerner / Purdue Extension
While fall is the time to plant hardy bulbs, it is also the time to dig up tender perennials to save for next year’s garden. Some flowering perennials are not hardy in our climate but produce an underground bulb, root or other structure that can be lifted from the soil and stored overwinter. The most common garden plants in this category include gladiolus, caladium, tuberous begonias, canna and dahlia. Although these plants are all collectively called tender perennials, each is best handled a bit differently.
Dig Elephant Ear/Taro before frost.
A number of tropical plants grown for foliage rather than flowers are collectively called elephant ears by many gardeners. Caladiums are grown for their brightly colored leaves, while Alocasia and Colocasia are grown for huge, waxy green leaves that are often highlighted with white, purple or another contrasting color. All of the elephant ear species are quite sensitive to cold temperatures and should be dug just before, or soon after, light frost. Cut the stems back to 3 to 6 inches, then place the tubers in a warm location for seven to10 days to remove surface moisture. To prevent excessive drying in storage, pack the tubers between layers of dry vermiculite, peat moss, sawdust or similar material in a strong box. Store at 50-60 degrees F (closer to 60 for Caladiums).
Tuberous begonias should also be dug just before frost. Cut the tops back to 2 inches and air dry the roots for two to three weeks in a warm location. Then, store in boxes, as you would caladiums, but decrease the temperature to about 45-50 degrees F.
Dig cannas after first frost.
Dahlias should be cut back to about 3-4 inches after the first light frost. Then, carefully lift the plants, leaving as much soil attached as possible to prevent breaking the fleshy roots. Because they are so susceptible to drying, dahlia roots should be air-dried for only a few hours or so. Then, pack in boxes, as you would caladiums, and store at 35-40 degrees F.
Gladiolus produces underground, compressed-stem structures called corms, which should be dug when the foliage just begins to fade, usually after a frost. Use a spading fork to carefully lift the plants, and save any of the miniature corms (called cormels). If planted next year, these cormels will grow larger and eventually reach a size that will support flowers as well as foliage.
Store canna rhizomes in boxes at 45-50 degree F.
The corms should be cured before storing to help prevent disease from developing. Cure the corms for two to four weeks in a warm (about 75-80 degrees F) room where air can circulate around the corms. Once cured, the corms should be stored dry in a cold, but non-freezing, location, about 35-40 degrees F. Old nylon stockings or onion bags hung from the wall allow good air circulation throughout storage.
Canna need not be dug until after a hard frost. Cut the tops back to 4 inches, lift with a spading fork and air dry in a warm spot for one to two weeks. Canna roots do not require covering they can simply be placed in shallow boxes. The roots are best stored at 45-50 degrees F.
Planting Tender Perennials
The words "tender perennial" sounds a bit like a contradiction. The group of plants considered "tender perennials" in Iowa can be overwintered in gardens in warmer climates. However, in order to keep these plants from year-to-year in the midwest, the rhizomes, corms, or tubers need to be dug in the fall and stored indoors over the winter months. A few plants considered tender perennials include: dahlia, gladiolus, tuberous begonia, canna, calla lily, and caladium. Below are some tips on planting and growing tender perennials in your garden. See the 1996 Horticulture and Home Pest Newsletter, page 157, for information on digging and storing tender perennials in the fall.
Dahlias (Dahlia hybrid) are beautiful garden flowers and make excellent cutflowers. They are available in a wide range of colors, flower forms, and plant height. Bedding dahlias that are used in containers or border plants in a flower garden, can be started from seed and are typically treated as annuals. The large-blooming varieties are grown from tubers or cuttings. Select firm healthy tubers with healthy buds. Plant dahlia tubers in warm, well-drained soil. Don't be in a bury an plant them too early or the tubers will rot in the cool soil. Select a full-sun location that is well-drained. Plant tall-growing dahlia varieties 2 to 3' apart. Drive a strong 6 to 7' stake into the ground where each dahlia tuber is to be planted. Dig a hole near each stake and plant the tuber, sprout-side up, 3 to 4" deep. The tuber should be about 3" from the stake. Dahlia varieties that grow less than 2 1/2' tall will not require staking and are planted only 2 to 3" deep. After the third or fourth set of leaves appear, pinch out the growing point. This will encourage new side branches to form. Begin tying the stem to the stake when it's about 18" tall. Use strips of old nylon stocking or twine to tie the stem to the stake. Keep dahlias well-watered throughout the summer and remove spent blooms.
Cannas (Canna x generalis) add a tropical appearance to annual and perennial gardens. The large green, red or striped foliage make excellent backgrounds, temporary screens or focal points in the garden. Tall scapes of bright flower colors bloom from mid-summer into early fall. Cannas are available in standard (3 to 6' tall) and dwarf (2 to 3' tall) varieties. Plant firm, healthy canna rhizomes a week before the average frost date in your area. Select a full-sun location that contains rich, moist soil. Plant the rhizomes horizontally about 6" deep and 18" apart with the buds upright. Keep the plants well watered and fertilize once or twice a month with a balanced liquid fertilizer. Remove spent blooms.
Tuberous begonias (Begonia x tuberhybrida) are perfect selections for containers or window boxes. Their large brightly colored flowers, ranging from white through yellow and orange to deep red, will brighten decks and patios. They prefer partial shade and a soil mix with a high organic matter content. A soil mix containing 1/2 and 1/3 part sphagnum peat moss will help hold moisture in the soil. Tuberous begonias are planted as firm, round tubers. Plant them in the containers about 2" deep with the rounded side down ("scooped-out" side up). Keep them will watered and do not allow the soil to dry out. Remove faded, spent blooms.
Gladiolus spikes (Gladiolus xhortulanus) brighten bare areas in perennial gardens in mid-summer and make excellent cutflowers. They bloom in nearly every color of the rainbow. Gladioli are planted as corms from early May through early July. Do not plant all the corms at one time. Plant a few every 7 to 10 days to provide a continuous supply of blooms. Plant the corms in a full-sun location between 4 and 6" deep, depending on the size of the corm. Space the corms about 5 inches apart. Keep them well watered. The flower spikes appear two to three months after the corms are planted and last about two weeks in the garden. It may be necessary to stake the flower spike to keep it growing straight and upright.
Fancy-leaved caladiums (Caladium x hortulanum) produce large, colorful leaves that give midwest gardens a very tropical appearance. The foliage patterns accent the veins with shades and combinations of green, white, red, and pink. They prefer a shady to partially shady location and a moist, well-drained soil. Caladiums can be purchased as plants that have been grown in a greenhouse or as tubers. Plant the tubers around the last frost date in your area. Space the tubers 2 to 3 inches deep and 4 inches apart. Water them frequently.
Calla lilies (Zantedeschia aethiopica) also have a tropical appearance and grow 24 to 36" tall. The flowers consist of a center spike (spadix) with a single petal (spathe) that curves around the spike. The flowers come in a wide range of pastel colors. The arrowhead-shaped leaves may be solid green or green with white speckles. Calla lilies are excellent as marginal plants in water gardens and are attractive in bog areas or along streams. They prefer partial shade but will tolerate full sun if given ample moisture. Calla lilies can be started indoors in March or April or planted directly into the garden after the danger of frost is past. The soil should contain ample organic matter to help retain moisture. Plant calla lily rhizomes 4" deep and 18" apart.
This article originally appeared in the April 25, 1997 issue, pp. 52-53.