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My Tree Dahlia (Dahlia imperialis http://davesgarden.com/guides/pf/go/56917/ ) grew this year very well with a stalk about 2 inches in diameter at the base and most of the way up the stalk, and lush green leaves. I think it benefited from the additional amount of sun it is getting after a diseased Live Oak tree was finally taken down by the city last year. However, when the Tree Dahlia reached about 10 ft tall, it started to lean like the Tower of Pisa. I intended to stake it up, but never found the few moments required to give it a support. Within a week or two after it started to lean, the Tree Dahlia eventually collapsed flat across the ground. I was concerned that if I tried to stand it erect again, the mostly hollow stem would snap at the base and it would be totally kaput for this growing season, so I left the stem to lay flat on the ground.
The growing tip of the Tree Dahlia made a 90 degree turn straight up and began to grow toward the sun. That portion of the Tree Dahlia is now at least 5 ft high and the flower buds are forming right on schedule and will be opening soon (and it will remain in flower until the first frost which usually comes in late December).
It has occurred to me that the toppling of the Tree Dahlia may turn out to be a good thing because it may provide me the opportunity to propagate more Tree Dahlias from the 10 ft portion of the stem that remains flat on the ground.
My plan is to dust the entire length of the stem with some rooting hormone, then cover the stem with good potting soil so that I build a mound of soil about 1 ft high that will bury the stem. I am hoping that new Tree Dahlia plants will spring up from the nodes of the stem and that the new plants will have the opportunity to root in the potting soil that is covering the stem.
Without any assistance on my part, two new Tree Dahlia plants have sprung up from the root crown and each of those plants is now about 2 ft high. I think that the rooting hormone and potting soil mound over the stem will encourage additional new plants from the nodes.
I don't think I have anything to lose by attempting this propagation experiment. My Tree Dahlia dies back to the root crown each winter and then emerges early in the spring around mid-March and typically sends up a single shoot from the root crown that grows to the final height of about 15 ft (unless it falls over as it did this year, or gets snapped off and doesn't get a chance to flower as it did last year due to a falling 40 ft limb from the city's diseased tree that crushed many of the plants in the same area of the Tree Dahlia).
I think about a foot of potting soil should be sufficient to keep the Tree Dahlia stem from freezing, but I may provide a blanket or twinkle lights or both on the coldest nights when the temperatures fall below 32 F for a few sustained hours on about a dozen nights between late December and mid-March.
Do any of you have any experience with propagating a Tree Dahlia from the stem in the way I will be trying?
All my previous efforts to propagate the Tree Dahlia from stem cuttings have failed. There were plenty of segments of Tree Dahlia stalk last year when the plant was crushed by the falling limb. I was hopeful that some of those stem pieces would root because the stem segments would send out a few new leaves, but then the stem piece would rot before any roots were formed.
I imagine the typical method of propagating a Tree Dahlia would be from root divisions, but I've not been successful in getting new plants from this method either. It may be that my Tree Dahlia (now about 5 years old) doesn't have a large enough root tuber to provide viable divisions.
I think if I can keep the buried 10 ft Tree Dahlia stem alive and keep it attached to the root crown (which will also be covered in about 1 ft of potting soil) without any rot occurring from too much moisture or soil organisms, I have a chance of getting several new Tree Dahlia plants that will arise from the stem nodes next spring.
Your comments, advice, suggestions or any information will be greatly appreciated!
The photo is of the root crown with two new Tree Dahlia plants currently about 2 ft high.