I raise 12 in a raised bed with no problem. I do use rebar to stake them up. Steve in Baltimore County.

It really depends on the varieties you're planting. If you're trying to grow 6' varieties, they'll need more space between them in order to get good flowering. On the other hand, if you pinch them or they're a bushier variety, they too will need more space between them.

I space mine according to their height.

Cheers,
Russ

So I'm still not sure what kind of spacing a 6' dahlia would need. My dahlias run the gamut of tall to dwarf. How much space should I give the ones that reach for the sky?

-Ray

I would put a 6' dahlia in the middle of a 6' diameter circle of free space.

If you are pinching them to make them bushier, add another 1' to the diameter of the circle.

This gives enough room for the plant to receive lots of light, air, and moisture. It also gives you space to get around it to take cuttings.

You can certainly fill that space with ground cover or tulips/daffodils or anything that will likely be cut down (or cut lower) by the time the dahlias are ready to bloom.

Cheers,
Russ

Can I use fir branches in the bottom of a raised bed for dahlias?

I have no idea why you would want to use fir branches in the bottom of a raised bed, so perhaps you could explain your thought a bit further.

The beds are 24" high and it saves on the purchase of soil.

Just contacted Swan Island Dahlias. They said fir boughs are ok for bottom filler as long as there is 16" of soil on top.

It's called Hügelkultur method. Google it or check utube.

Well, I am no expert on Hugelkultur, but from what I have read you seem to be trying to use a steam shovel where all you need is a shovel.

Hugelkultur would appear to be an interesting horticultural feature, but given how old the practice is, and how rarely it is actually employed, I’d say there must be sound reasons for it. Let’s consider a few points:

1. Erosion: Most mound pictures show the sides as being 45 degrees or so. Manipulation of the mound, say to plant or extract tubers, is going to have a significant impact on the structure, and the layered composition of the mound. I’d say that planting and extracting dahlia tubers is not going to be fun, or easily done.

2. Collapsing due to watering and/or decomposition: Your fir branches are supposed to decompose, and will do so, presumably causing the mound to collapse into the voids. Will this result in newly planted tubers or growing tuber masses being enmeshed in those branches? The same can be argued regarding the watering of the mound. Whether it happens naturally due to rain, or artificially, pockets will eventually form.

3. Dahllias most certainly do not need deep soil beds, and do not use such deep soil, so how do they tap into the nutrient richness of the buried goodies (e.g. rotting branches). To what extent will whatever else you plant on the sides of the mound remove or consume the nutrients in the bed? You will have to ensure that nothing you plant on the sides of the mound attract anything that might harm the dahlias (can you say “spider mites” due to the excessively available moisture?) You are likely going to have to plant dahlias in a single row down the middle of the mound.

4. Meanwhile, dahlias do just fine in pretty much any garden soil. Heck, in my raised beds at my forest farm I used the crap forest floor soil mixed with purchased garden soil to great effect.

So I would say that for dahlias, Hugelkultur is not needed, but good luck.

Cheers,
Russ