QUESTION: I found your article on planting spring bulbs quite interesting. Some years back, I thought I read an article that said dahlias didn’t do well planted inside and then transplanted outside later on. But in point of fact, they don’t produce well at all being planted outside in late May. Could you give me more details on when and how to start them inside?
ANSWER: Thousands of dahlias are planted in one and two gallon nursery containers each year by wholesale growers. They are typically planted in artificial soil mixes for container plants similar to what is available in garden stores.
Dahlia bulbs are actually long tubers with a growing point or eye at the top. They are planted with the eye of the dahlia an inch or two below the top of the pot with long tubers further into the pot. They are grown in greenhouses where the minimum temperature is about 60 degrees. They are typically moved outside in May. They are then sold at various stages, but mostly when starting to bud and bloom in mid-summer. Because the temperature is warmer in the early stages of growth, they mature sooner and begin blooming in mid to late June.
Dahlia tubers can be planted similarly in the home. They do not need light until bulbs have sprouted. Room temperature is good for sprouting. After that bulbs need as close to full daylight as you can provide.
You can start bulbs inside without direct sunlight or artificial light. Plant bulbs as described above in mid to late March. It will take two to three weeks for the bulbs to sprout and grow above the soil if started at room temperature.
Start putting the pots outside in as much sun as possible after sprouting whenever the temperature is above 40 degrees. Bring them back in at night. The combination of cooler day temperatures and warmer night temperatures keeps plants more compact. After a while they can be left outside continually unless frost is predicted. Transplant outside in late May or early June
Tuberous begonias can also be started inside in smaller pots or hanging baskets. They are shade tolerant and will grow inside with strong indirect light. Wait until temperatures are above 50 degrees before putting them outside in the daytime. Place them in a shady area outside.
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