The book says you need support for corn which grows about 6' tall so I would apply the same support principles that are recommended in the book for corn to your sunflowers.

If you do not have a bottom on your bed, I would not worry too much about the soil depth. Where I am sunflowers grow wild in our very, very poor soil so they will likely find their way to what they need if the 6 inches is insufficient.

How much space would you use for each seed?

Mine are 12" deep, so I can't help with the depth issue. But as for the spacing, we planted one per square. The one in my son's box is over 6' tall and is wide enough to take up the whole square. Even the shorter ones send out their leaves quite wide.

How much space would you use for each seed?

Spacing for sunflowers is on the package. It reflects the flower size.

Dan

I agree with tishtoshnm.

One of the other problems that I would expect with Mel's Mix for sunflowers is one of the greatest benefits of Mel's Mix - the friability. The taller the plant, the greater the need for a strong medium for the roots to anchor in. Mel's Mix is too friable to allow the roots alone to hold the plant in place, which is why he recommends support. In my own opinion, if you had a clay soil under your box, and it's open, the roots will work their way down and into the clay to support themselves. The plant won't get taller than it can support if allowed to grow on its own. The sunflowers should be able to anchor themselves if the depth of soil is available. 6 inches of Mel's mix, and probably even 12 inches, isn't going to be enough to support a plant over 6 feet tall without help. "Natural" soil on the other hand should be able to handle it with no problem. I keep my boxes open on the bottom, so that I don't have to concern myself with these details! ;-)

Good luck.

Most of the roots of a sunflower are shallow. Here in Texas they have to be supported as then we get a heavy rain they fall over.

It may depend on the size of the flower, not just the size of the plant. The ones that grow wild here hold up well to the wind and rain during the monsoon season. The soil is clay and the flowers are on the smaller size.

I agree about the friability of the Mel's Mix and support. I am considering adding my heavy soil the to the tops of my beds to help support the plants in the wind.