Starting out -- I recommend starting the seeds in small pots around 2-4 inches (5-10 cm) tall. The pots should have drainage holes.
Tree Dahlia likes well-draining soil. A good mix is 2 parts potting soil to 1 part perlite or coarse sand.
Fill each container with the soil mix and water it until it is evenly moist, but not soggy. Place a seed on top and sprinkle a thin layer of soil on top - just enough to barely cover the seeds, because exposure to light improves germination. Lightly water the top soil layer.
Until the seeds sprout, ensure that the soil surface doesn't dry out. You may enclose the pots in a plastic container or bag to maintain moisture, but leave it open a crack to let in fresh air.
The ideal temperature for germination is about 65-75 degrees F (18-24°C). If you use a heating mat, check the soil temperature to ensure that it doesn't overheat.
Place the containers in a bright spot out of direct sun. A fluorescent bulb kept right above them provides the right amount of light.
The seeds should start sprouting in 6-8 weeks, but allow up to 3 months for any slow ones to sprout. Once they sprout, give them bright light, but protect them from direct sun the first 3-4 weeks.
Watering -- Tree Dahlia likes evenly moist (but not soggy) soil. Don't let the soil dry out, but don't keep it perpetually wet either.
Fertilizing -- If your potting soil contains fertilizer, your seedlings shouldn't need feeding the first 4 weeks. Then feed about every 4 weeks with an all-purpose fertilizer at 1/8th the recommended dosage. When the seedlings are 2 months old, increase it to 1/4 strength, and then 1/2 strength at 1 year old.
It's normal for an older leaf to occasionally turn yellow and drop, but if it seems excessive, the plant may need more nitrogen fertilizer. The soil could also be too dry down in the root zone.
Transplanting -- When the seedlings are 2 months old, repot to a larger container, about 1 quart (1 liter) in size. Water the soil before repotting to keep the soil ball from breaking apart. For the first week after repotting, give shade, and no fertilizer.
Climate -- Tree Dahlia is hardy to Zone 8, but it's known to flower in Zone 7b in warmer years (frost may ruin the flower clusters). The tubers are said to be hardy to 20-25 degrees F (-6°C) if well-mulched. Protect the plants from strong winds, as the young shoots and flower stems are fairly brittle. You may need to stake your plant if your area is windy.
Planting out -- Once the plants are 2 feet tall they can be planted in the ground. If you're going to keep them in a container, repot to a larger one after a few months.
Pruning -- If for some reason you don't want a giant-sized plant, you can keep it shorter by pruning the tops a few times throughout the season. Pruning the top will make it bushier.
If you have any questions or problems, please email me.
Enjoy your plant!
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