Plant Wise Malvern, IA 51551 712-624-8616
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Mills County Extension 61321 315th St; P.O. Box 430 Malvern, IA 51551 Plant Wise 712-624-8616 http://www.extension.iastate.edu/mills Prepared by: Shirley D Miller, Mills County Master Gardener Coordinator and Mills County Horticulture Program Assistant July 2021 Dahlias and their Beauty Dahlias belong to the Asteraceae (Aster) Have you ever tried to grow Dahlias? They are so family along with daisies and sunflowers. diverse in color, size and style! They are Cavanilles noted three dahlia cultivars Dahlia considered a tender perennial in cold regions of pinnata, D. rosea and D. cochinea. Now there are North America. They are only winter hardy in 5,692 records (including species and cultivars) on planting zones 8 to 11. Gardeners in zones 2 to 7 the RHS’s horticultural database, with new cultivars can simply plant dahlia tubers in the spring and constantly being bred. either treat them as annuals or dig them up and Dahlias range from dwarf plants ideal for bedding, store for winter. Dahlias love moist, moderate to giants such as Dahlia imperialis found growing in climates. Dahlias brighten up any sunny garden the wild from Guatemala to Colombia where it can with a growing season that’s at least 120 days long. grow upwards of 6m tall. Some cultivars have flowers that reach beyond dinner plate size at 14 inches across. They are great plants to grow in a cut flower garden and their beautiful flower heads often last well into WHEN TO PLANT DAHLIAS the autumn, extending the flowering season. In the spring, don’t be in a hurry to plant; dahlias WHERE THEY COME FROM will struggle in cold soil. Ground temperature They were discovered in the 16th-century by should reach 60°F. Wait until all danger of spring Spanish botanists who noted dahlias growing wild frost is past before planting. A good rule of thumb is on the hillsides of Mexico, although paintings and to plant them a little after the tomato plants go in. illustrations suggest that native civilizations had Some gardeners start tubers indoors in containers been cultivating and breeding them much earlier. a month ahead to get a jump on the season. Dahlias were originally grown as a food crop for Medium to dwarf-size dahlias will do well in their edible tubers. Gardeners suggest they taste containers. like a cross between carrot, celery and potatoes. CHOOSING A PLANTING SITE Dahlia coccinea was one of the first species to be Select a planting site with full sun. Dahlias grow introduced into Europe from Mexico in 1789. Abbe more blooms with 6 to 8 hours of direct sunlight. Cavanilles (director of Real Madrid Botanical They love the morning sunlight best. Choose a Garden) named the Dahlia after Anders Dahl, the location with a bit of protection from the wind. Swedish botanist. Dahlias thrive in rich, well-drained soil. The pH level of your soil should be 6.5-7.0, slightly acidic. mulch harbors slugs and dahlias like the sun on If you have a heavier clay soil, add in sand, peat their roots. moss to loosen the soil texture for better drainage. WATERING Large dahlias and those grown solely for cut There’s no need to water the soil until the dahlia flowers are best grown in a dedicated plot in rows plants appear; in fact, overwatering can cause on their own, free from competition from other tubers to rot. After dahlias are established, provide plants. Dahlias of medium to low height mix well a deep watering 2 to 3 times a week for at least 30 with other summer flowers. minutes with a sprinkler. Dahlias need lots of water. Like many large-flower hybrid plants, the big dahlias may need extra attention before or after rain, when open blooms tend to fill up with water or take a beating from the wind. FERTILIZING Dahlias benefit from a low-nitrogen liquid fertilizer such as a 5-10-10 or 10-20-20. Fertilize after sprouting and then every 3 to 4 weeks from mid-summer until early autumn. Do NOT over HOW TO PLANT DAHLIAS fertilize, especially with nitrogen, or you risk small Avoid dahlia tubers that appear wrinkled or rotten. or no blooms, weak tubers, or rot. Pink “eyes” (buds) or a little bit of green growth are PINCHING, DISBUDDING AND DEADHEADING good signs. Don’t break or cut individual dahlia When plants are about 1 foot tall, pinch out 3-4 tubers. The smaller flowering types, which are inches of the growing center branch to encourage usually about 3 feet tall, should be spaced 2 feet bushier plants and to increase stem count and apart. The taller, larger-flowered dahlias should be stem length. If you want to grow large flowers spaced 3 feet apart. If you plant dahlias about 1 try disbudding - removing the 2 smaller buds next foot apart, they make a nice flowering hedge and to the central one in the flower cluster. This allows will support each other. the plant to put all of its energy into fewer but The planting hole should be slightly larger than the considerably larger flowers. Also deadhead as the root ball of the plant and incorporate some compost flowers fade. This encourages more blooms. or sphagnum peat moss into the soil. It also helps WINTER CARE to mix a handful of bonemeal into the planting hole. Dahlia foliage dies back with the first light frost in Otherwise, do not fertilize at planting. fall. The tubers should be dug up before the first Dig a hole that’s about 6 to 8 inches deep. Set the hard freeze and stored indoors. Store them like tubers into it, with the growing points, or “eyes,” your other flowering bulbs, in a cool dark area. facing up, and cover with 2 to 3 inches of soil. As Periodically check them throughout the winter. the stem sprouts, fill in with soil until it is at ground level. Squash Vine Borers Have you had that sick feeling when you go to your garden and see your squash plants wilted down and dying! Most likely it was caused by squash vine borer larvae. CARING FOR DAHLIAS Tall, large-flowered cultivars will require support. Place stakes (5 to 6 feet tall) around plants at planting time and tie stems to them as the plants grow. Dahlias start blooming about 8 weeks after planting, starting in mid-July. Do not water the tubers right after planting; this encourages rot. Wait until the sprouts have appeared above the soil to water. Do not bother mulching the plants. The 2 Squash vine borers (Melittia satyriniformis) are 1. Apply Bt (Beneficial Bacteria Spray). the larvae of an attractive orange and black sesiid Apply an OMRI certified Bacillus thuringiensis (aka moth that is active during the day. Due to their “Bt”) spray on the leaf stems and main stem of the similar appearance, many people mistakenly plant (not the flowers). assume that adult squash vine borer moths are in Bt is a naturally occurring bacterium found in soils the wasp family. They’re not – they’re moths. Like around the world. Research indicates that it is squash plants themselves, squash vine borers are virtually harmless to non-target species. If you also native to America. Adult squash vine borer spray Bt on the stems and leaves of your squash moths are pollinators. However, they lay their tiny plants once per week, the squash vine borer’s eggs copper-colored eggs on squash plants. or larvae will be killed before the larvae is big enough to harm your squash plants. 2. Mulch the squash plant’s stems. Keep the main stem of the plant covered with mulch as it crawls along the ground. This method helps prevent the adult Squash Vine borer moths from laying eggs on the stems. You can even wrapping the stems of your squash plants in aluminum foil if you do not want to mulch. The point is to hide the main stem from the Squash Vine After 9-11 days, squash vine borer eggs hatch, at moth. If you go this route, be sure to routinely which point the larvae chew their way inside the check the rest of your plant since borers also lay squash plant’s stem and slowly eat the plant to eggs on the leaf stems or undersides of the leaves. death. A single adult female squash vine borer Squash vine borer eggs are small, round, shiny and moth can lay up to 150 eggs in a season! copper-colored, very similar in appearance to How can you tell if your squash plant has been “squash bug” eggs. The difference is that squash killed by squash vine borers? bugs lay multiple eggs in very tight clusters, while Basically here’s how. One day you have a healthy borers will often only lay a single egg in one spot - squash plant. 24-48 hours later, you walk outside to or if laying multiple eggs, the eggs won’t be tightly find a squash plant that is limp and collapsed. clustered. Check for small holes in the leaf stems and/or the If you see either type of egg on your squash plant, main stem of the plant with brown frass around the remove them by hand immediately and smash opening, indicating that you have squash vine them (or drop them into a cup of soapy water. 3. borers! Usually, once the squash plant has gone Grow resistant squash varieties. limp from damage, there is no way to bring it back. Most summer squash varieties are very susceptible This experience can be heart-breaking.