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Growing Dahlias in Anchorage, Alaska
By Amelia Walsh, Master Gardener
Copyright© Amelia Walsh
Native to the mountainous regions of Mexico, the genus Dahlia was named in honor of Anders Dahl, an 18th century Swedish botanist. From just a handful of original species Dahlias have been hybridized over the centuries into an amazingly diversified genus with plant sizes from 1 ft to 7 ft tall and flowers in almost any color imaginable, except clear blue, and blooms ranging from 2 to 12 inches across.
When to Plant
Dahlias are planted in the spring after the threat of frost has passed, soil temperature is at least 55 degrees F and plants have been hardened off. These steps are very critical because of the tender nature of the plant. Place plants in a shady location and protect them from winds and hot sun that will burn the leaves. To get a head start in the spring, after digging tubers the previous fall, divide and plant in containers and store for the winter. Newly purchased tubers in the spring should be planted in containers indoors under lights seven to eight weeks before planting outside.
Where to Plant
Dahlias are sun lovers. Sunlight is important to growth and flower production. If planted in a shady location expect less blooms and taller, spindly plants. Low growing and dwarf varieties can tolerate less sun.
Dahlias need well drained soil. If you have heavier soil, consider adding coarse sand or steer manure to improve soil texture for better drainage. Steer manure is the best soil additive as it adds nutrients while lightens the soil. For container grown plants, potting soil is recommended over garden soil. My favorite is Miracle Grow.
Plant in moist, not wet, soil. If you are not sure which side of the tuber is up, plant horizontally about 6" deep. If the eye is clearly visible plant them with the eye pointing up, adding a handful of bone meal to the soil. Do not water after planting; wait until shoots have emerged above the soil level. If you are planting your Dahlias in the garden space 12 to 24" apart. Dahlias do very well in containers. If you will use containers plant tubers half way down and add more soil as the plant grows. If tubers are planted too deep they could rot and if planted too high the stalks could break as the plant grows. Be very diligent about watering. Remove any saucers that will collect water. Place any type of small hole mesh (the type that spring bulbs come in or similar) over the drain holes to prevent soil to escape and, hopefully, keep slugs and other pests from entering.
Dahlias tubers do not need to be watered when first planted. After shoots emerge above ground, a deep watering that reaches the bottom of the tuber is best. Adjust your watering to weather conditions and the size of the container, if grown in pots. Soil should be slightly damp and not soaked. If tubers are kept too wet they will rot.
I use a onetime application of slow release fertilizer when plants are ready to be placed under lights along with sterilized steer manure. Otherwise a low nitrogen fertilizer like those used for vegetables, such as a 5-10-10, 10-20-20 or similar combination works also. If you choose the latter one, apply after your plants are established and repeat every 3-4 weeks, following manufacturer's directions. The use of compost and high nitrogen fertilizer is discouraged because they promote foliage, spindly plants and fewer blossoms. Do not overfeed.
Your plants will look better and be more manageable if you provide some type of support. Staking is highly recommended if you live in a windy location. Tomato cages, bamboo stakes, metal rods and or any other type of creative support you can provide, all will work very well. Tie stalk to the rod at time of transplanting and keep on doing it as the plant grows. Maintaining your plants so that they will grow upright will ensure strong growth and prevent blossoms from breaking. Staking is a must for dinner plate Dahlias that have big, heavy, beautiful blooms.
Plants that have blooms removed regularly will produce more blooms. The more you cut, the more they bloom. Dahlia blooms generally form clusters of three. Some growers choose to remove the two side buds to promote a larger terminal bud. This is strictly a personal choice. If you choose not to disbud, at least dead head on a regular basis. This again will promote more blooms.
Pinching promotes the growth of bushier and stronger plants. It does not delay the blooming time and it will encourage more blooms. After the plant has developed at least three sets of leaves, pinch out the center shoot just above that third set of leaves.
Digging From the Garden
Since our weather conditions are different than most other parts of the country, our Dahlias have to be dug up before they are ready. Ideally you should wait for the first frost to turn leaves brown and black to dig them up. Realistically here it is done either toward the end of September or the beginning of October according to how the season is progressing. Cut off the stalk leaving a 4 to 6" stem, and gently lift tubers with a spade or garden fork. Be very careful not to damage the neck or the protective skin of the tubers. Shake off excess soil and let dry for 2 to 3 weeks. This will allow the protective skin around the tuber to harden and seal inside enough moisture for the winter. At this point you can either store the tubers for winter or divide and store them.
Dividing and Storing
This process can be done in the fall or in the spring. I divide in the fall. Using a sharp knife (or a flat head screwdriver), remove any damaged tuber and the hair roots. Tubers should have an eye to grow; they are located near the stem of the clump. Not all tubers will have an eye visible; many times they appear during the overwinter storage period. If they are too difficult to locate, divide clumps in halves or quarters before dividing into individual tubers. Plant tubers in appropriate size container, (my favorite way to store for winter) label and store. If you have a different way of storing and it has worked for you in the past do not change a good thing. Container soil should be slightly moist, not wet. Ideal storage temperature is 38 to 50 degrees F. If tubers are kept too warm they start to grow or could wrinkle and shrivel. If they start to grow too early, simply pinch them back. If tubers are kept too cold they could freeze and if stored too wet they could rot. Check on them throughout the winter.
Storage Container Grown
If you are not planning on dividing the tubers grown in containers, cut off stalks to about 4 to 6". Let soil dry and store in cool, dark place. Do not water.
Advantages of Growing in Containers
Plants can be easily moved around to decorate your front or back porch or to adjust your display with taller plants in the back. In case of high wind, they can be moved to a protected area. If they are still in full bloom but the weather is turning too cold, or in case of early snow, they can be moved and enjoyed indoors.
Pests and Diseases
The most common pests for Dahlias are: aphids, spider mites, slugs and thrips. Aphids will curl leaves and leave a sticky film on the foliage. Control by spraying with water or insecticide. Spider mites produce a fine web on the foliage; they can be controlled by a forceful spray of water three or four times a week or spraying with a solution of dishwashing soap at the ratio of 1 tablespoon of soap to a quart of water. You can also use a miticide. Dahlias are magnets for slugs especially in a wet and cold spring. Apply slug bait when you first plant and during the season if necessary. Thrips are very minute and are barely visible on the blossoms except for the white blooms. They are pollinators in the wild. Unless they really bother you, or you are entering your plant in a show, you can ignore them. When using insecticidal soap or commercial pesticide, always follow label directions and use protective gear. Diseases include powdery mildew, tuber rot and Botrytis blight. Powdery mildew will appear with poor air circulation. It covers foliage with a whitish-gray powder; after the plants are several feet tall remove lower leaves to increase air circulation. It will not kill the plant. Botrytis blight is a fungus disease and appears as gray-fuzzy spores on the foliage. Management of environmental conditions, good cultural practices and a fungicide will take care of this disease. Tuber rot is caused by overwatering. Growing plants with organically enriched soil will prevent most pests and diseases. Whenever you can, grow organic.