They are sensitive to root rot, yes; and it is easy to do this in the home garden. The soil must be rich as delphinium are gross feeders. (Mulch and sheep manure are recommended by good delphinium growers and breeders).

PH neutral soil preferred, 6.0 - 7.0; but successful over a wide range of soils.

They need well-drained soil, particularly in warm areas: they must not get very warm, very wet feet together.

Also, they should have about 1 square yard space apart from one another. This sounds like a lot, but this is the best situation for them.

I had also read somewhere that they like a bit of lime in their soil to make ure that th e Ph is right. However, now.

Keep as cool as possible with thick mulch (sawdust also helps prevent slug and snail attack).

Detailed growing tips for the Stock Vintage can be found at the attached link. Geared more towards early production from seed, rather than finished growing, but you may be able to pick up some clues to your problems.

Here is a link that might be useful: Vintage Stocks

My friend, you are fighting a losing battle. I have heard of people in Atlanta growing delphiniums as annuals with limited success, but you're even further south than that. Our hot, humid climate, especially nights, are the major problem for both delphiniums and stock.
If you just have to try (I did :), you should start the plants from seed and plant them out in the fall. They will only bloom for a very very few weeks in spring, since once the hot weather moves in, they're done. I found that they often don't have time to get large enough in the spring to bloom before the heat arrives.

As a substitute, I plant larkspur from seed (right on the soil) in the fall and it gives me beautiful, spiky colors reminscenet of delphiniums in late spring when the irises and roses bloom. They will reseed reliably. (Nichols nursery sells larkspur in packs of just one color, which is nice.)
For spiky blooms in summer, I rely on salvias, primarily. This year I plan to try eucomis (pineapple lilies) and try (again) kniphofia. No, they're none of them as satisfying as delphiniums would be, but my gardens are still lovely.

As donnabaskets says, delphiniums don't come easy down south. Took me about five years to get the first batch to grow. Some of my thoughts and observations ...

Seed needs to be fresh and kept in the frig until time to plant. Buy a pack that has sat on a store rack for a couple of months and you have zero chance. Use a good peat based medium. Pat the seed into the surface then dust very lightly with powdered line, just enough to give the surface a bit of a gray look. Cover with an eighth inch of loose medium. Keep the flats moist in an air conditioned room out of light. This should be done in August.

Don't think you will keep the plants over the summer. It seems to be the high soil temp that kills them. I've seen them survive 98 in late June, but as the soil heats up in July it's a hopeless case. If I grow some more I may try digging them after the blooms have been cut back a week or two and storing the roots in a frig.

I like the large Pacific Giants which can produce 3 to 4 ft spikes in the south. The shorter ones are just not worth the trouble.

If you can get them to survive the seedling stage to be planted in fall they are almost certain to go on to bloom. Some years I loose them all. A good year and I get half to the garden. You want easy, do larkspur, but they aren't going to have the big beautiful blooms that come on delphiniums.

{{gwi:3649}}

Thanks y'all for the feedback.

Mind you I didn't start the delphiniums or stocks from seed. I bought them from a local nursery. I planted them as annuals. They will be removed in April.

So for now, I think I'll start feeding them on a weekly schedule and see if that fattens them up.