Is that sick Delphinium in the pot? Are you comparing Delphiniums in the ground with those in rusty pots? If so it doesn't appear to have much soil to speak of. And how long has it been in the same medium?

never heard to spraying plants with hydrogen peroxide to avoid fungus...

nor spraying prophylactically for fungus..

ken

Spraying prophylactically for fungal infections is standard practice.

As for Hydrogen peroxide, it is widely used, but not with much scientific basis. Are hydrogen peroxide products effective fungicides? - Vegetable Growers News

Just outof curiosity

is there scientific evidence that diluted 1 or 2% milk is a fungicide or effect mildew treatment?

i was very surprised that it appeared to work on my musty phlox though I didnt bother to quantify how.

Marie, I had a few roses terribly covered in mildew and used a fine misting of undiluted 2% to completely cover the foliage. I believe I had done so later in the day and not in direct sun and no rain was received for several days following. Well, I got a complete wipe out of that mildew with foliage looking healthy as can be soon afterwards, new foliage stayed pristine as well.

Yep it is worth a half a-cup milk!

and I’m convinced that, in rhe absence of hard data, magical thinking, crossing fingers and clicking your ruby slippers together 3 times increases the sucess of most homegrown interventions.

Obviously, Marie, if you failed to click the ruby slippers under the light of a full moon (but never on a Friday), success will remain forever out of reach.

To floral_uk. No, it's not in a pot. What you see is an old copper collar around a slice of tin can, to repel slugs.

Right. Got it.

Hoping for an answer to the OPs question. I have some larkspur grown from seed, and those seedlings won’t budge at all. They shouldn’t stay 4” tall for four weeks!