Dahlia is a genus of bushy, summer- and autumn-flowering, tuberous perennials that are originally from Mexico, where they are the national flower. There are literally thousands of cultivated varieties of Dahlias which have been hybridized throughout the years. Dahlia plants range in height from as low as 12" (30cm) to as tall as 6-8 feet (180-240cm). The flowers can be as small as 2" (5cm) or up to a foot (30cm) in diameter.
Dahlias should not be planted until all danger of frost has passed, and the soil temperature reaches 58-60°F (14-15°C). Dahlias need a sunny location to thrive. An area that receives at least 8 hours of direct sunlight is best. Exception for hot climates, they will need morning sunlight, afternoon shade.
Excessively wet soil may cause the tubers to rot, so if your weather has been wet, you may want to wait for a drying trend. Dig and prepare a 12" (30cm) diameter by 12" (30cm) deep planting hole. Mix a shovel full of compost, a handful of bone meal, and a little Dolomite lime to the soil which was removed.
Fill the planting hole with the soil mixture until it is about 6" (15cm) deep. Then place the tuber horizontally in the bottom of the hole with the eye pointing upward. Tall varieties will need staking, so this is a good time to set an appropriate size stake into the ground next to the tuber. This will prevent damage which can result if it is added after the tuber has begun to grow.
Cover the tuber with about two inches of your soil mixture and water thoroughly. When the sprout begins to emerge from the soil, gradually add more soil mix until the hole is entirely filled.
Once the plant attains sufficient height, secure it loosely to the stake. Add more ties as the stem grows until the plant is supported approximately 24" (60cm) below the eventual top of the plant.
A Dahlia in bloom is a heavy feeder, so you may want to consider using a high percentage potassium and phosphorous fertilizers such as a 5-10-10, 10-20-20, or 0-20-20. Avoid Compost, Fish fertilizers, and high nitrogen water soluble types as they promote weak stems, small blooms, or no blooms, and tubers that rot or shrivel in storage.
Dahlias which have been started in pots may be planted in the prepared hole following the same procedures you would for any other perennial plant.
Most areas have enough rain to fill dahlia water needs until the sprouts appear above the ground. After dahlias are established, a deep watering 1-2 times a week is necessary during warmer, dryer weather. Hotter climates will need to water more often as conditions require. Proper watering promotes better blooming.
To promote a compact, bushy growing habit, with more flowers, pinch back the new growth when your dahlia is about a foot high. If your goal is to produce massive sized flowers, remove all of the side buds at the end of each branch throughout the growing season. If you want your dahlia to provide a continuous, extended flower show, you will need to remove the spent buds promptly.
Some gardeners choose to leave their dahlia tubers in the ground over the winter. In certain regions, this can be very risky but if you have decided to follow this path, make sure that your Dahlias are growing in very well drained soil and apply a minimum of 6"-12" (15-30cm) of mulch to the planting area before the ground freezes.
The best plan is to dig the tubers and store them in a cool, dry place for the winter. Prior to digging, your dahlias will need about a week to produce new sprouts on the tuber. The production of these new eyes can be stimulated by cutting the stem back to a 6" (15cm) stub, or will occur naturally when the majority of the plant has died back due to frost.The tubers will be easiest to divide if they are harvested after this one week period.
Using a garden fork to prevent damage, dig a circle about 12" (30cm) around the plant stub, and lift the clump carefully out of the ground. Use a gentle spray from your hose to clean and remove the remaining soil from the clump. Allow the clump to dry for a day in a cool dry place. You are now ready to divide the clump, then store the individual tubers or store the clump and do your dividing in the spring.
To produce a new plant, each tuber must have an eye, which appears at the point where the tuber connects to the main stalk. Using a sharp clean knife carefully separate tubers. Discard any damaged tubers and any that don't contain an eye. Place the tubers in a bed of sawdust or vermiculite, inside a cardboard or a wooden box.
Store them in a dry area where the temperature will remain at about 40°F (5°C).
Check your tubers periodically during the winter for signs of shriveling (moisten the storage medium), or mildew (treat with a dry fungicide such as Captan)
Best time to cut your flowers is in the cool mornings. Place the cut stems in 2-3" of very hot water (approx. 160-180°F, 70-80°C) and allow to cool at least one hour. This will set your blooms and make your flowers last for 4-6 days. Removing old blooms will keep your plants strong and blooming late into the season.
Generally, dwarf varieties of dahlias are started as bedding plants and then treated as annual plants. They will produce tubers during their first year of growth which may be dug and replanted in the same way as larger dahlias, but because of the ease of growing them from seed, most gardeners prefer to discard the old plants and start fresh the following year. Growing dwarf dahlias from seed can also reward you with surprising new hybrid varieties and colors.
Sow the seeds about 1" (3cm) apart in a seedling tray or individually in 4" (10cm) pots, and cover them with about ¼" (½cm) of fine soil. The seeds will germinate best at between 70-85°F (20-30°C). Keep the soil damp, but not soggy. Typically the seeds will sprout in 1-3 weeks. When the young plants have two or more sets of true leaves carefully transplant them into 4" (10cm) pots. Grow them in bright light, but not direct sunlight until it is time to move them into the garden.