Hi Dave,

The main disadvantage of clay for zinnias is that zinnias like to have well-drained soil that gives plenty of air exposure to their roots. The tight structure of clay doesn't give zinnias the air they want on their roots. I am puzzled that you say your clay doesn't retain moisture well, because all of the clay soils I have had experience have been quite good at retaining moisture once they manage to absorb it. A quick rain tends to run off before the clay has time to absorb it. I don't know of any zinnia varieties that would actually like dry clay.

" I also prefer mildew resistant varieties."

The most popular mildew resistant zinnias are the Profusions, the Pinwheels, and the Zaharas. I don't like those because they have small flowers and little plants. I prefer the big zinnias that aren't "mildew resistant". Zinnia mildew is mainly a problem in the Fall for me, and in the Fall I am saving seeds from my zinnias. For that reason, mildew isn't a serious problem for my zinnias.

I haven't grown them myself, because they also have small flowers on short plants, but the Zinnia Crystal Orange are probably tolerant of poor soil. There is also a yellow and a white variety available of Zinnia Crystal.

The soil here is dark silty and a bit clay-like, but it retains water amazingly well. I prefer to garden in sandy loam, so I have added several truckloads of sand here. My zinnias love the sandy loam, and so do I. If you don't have a trowel handy, you can just dig a hole in it with your bare hand.

ZM
(not associated with any product or vendor mentioned or linked)

Martha

A layer of mulch will facilitate water infiltration rather than rapid runoff
but you'll need to water thoroughly. Most people simply don't have the patience to provide enough water at any one time to be of much benefit to the root zone of their plants. If there's a mulch layer, the water needs to be of a long enough duration to get to the soil! The typical irrigation system can be practically useless when it comes to deep watering.

I don't grow a lot of zinnias, but second what ZM said about the little Zinnia angustifolia (the Ctystal series). Though they only come in white, orange, and yellow they are sturdy little workhourses in the garden, blooming nonstop until the firsf heavy frost with no fading, no summer doldrums, no disease, no deadheading required. The Crystal White is a must for my borders, so bright white all spring, summer, and fall.

I'd also recommend the Profusion series for sturdiness.

Kerp adding a nice mulch layer every year. I don't like to see those with clay soils turning or tilling. That activity can ruin soil structure which is soooo important with clay soils. If you dress the soil with compost, wood chips, fallen leaves that have been run over with a mower, strewn grass clippings (no weeds) on a routine basis you will be amazed at how rapidly it disappears into the soil. Worms and other macro and micro organisms work non-stop in their efforts without any help from us.

The ornamental bark chips don't do much to improve the soil, which is why I suggest wood chips instead of bark.

I do turn over some of my soil (perhaps my bad) working in fall leaves and compost. I compost as much as I can. It takes years to improve soil substantially. Soil can consume lots of compost or leaf mold. Maybe 3" to 4" thick layer of compost if you turn over soil 12" deep every year. You almost can't over do it. In my opinion.

This last year, I grew both Profusion Zinnia and Crystal Orange Zinnia (out of dumb luck). They both did well for me in spite of the soil. The location was a "first year garden" as I had to remove the sod. I don't think there was any mildew on them. I recall lots of flowers all summer long with minimal watering.

[I've been saving seeds for a while. It amazing how many plants you can get when you have seeds to spare. Wow!]

I mulched my zinnias with hardwood chips and they did great in my clay, while the uncovered clay areas down the row a foot or two were baked solid several inches deep.