H. s. 'Kurenai' is a white-flowering cultivar, so soil pH should have little effect on color. As the flower matures, it becomes pink and finally red, thus the name; Kurenai(Jp)=crimson(Engl). More sun exposure will produce a deeper color and in full sun, the sterile petaloids will become a deep red.
They are a small plant, attaining a height of about 3 ft. and I use them as potted plants in various places around the garden.
Another small H. serrata that I grow in pots, is the cultivar 'Little Geisha' ('Shiro Maiko'?), which is a white mophead.

I just ordered a 'Little Geisha' this weekend, and planted a Preziosa this spring. I think I might have discovered a new addiction... So, aside from Preziosa and Little Geisha, which of the h. serratas give you the biggest show and most color? I've seen pictures of various types that look like they are covered with flowers, and other pictures of the same variety only have a few flowers. Which types have flowers that show up well from far away?

Thanks!

Lois in PA

The amount of light, either direct sunlight or bright reflected light, influences the quantity of flowers produced. A location with several hours of morning sunlight would be ideal, I suppose, but isn't always possible. I have some, that receive up to 8 hours of afternoon sunlight, but moisture requirements are much greater. Use
of fertilizers, with a high Nitrogen (N) content, will produce lush foliage, but few or often, no flowers. I use none, only an annual application of compost and leaf mold.

Some of my best flowering H. serratas are:
'Woodlanders' - seed strain from Chollipo Arboretum, S Korea, introduced by Woodlanders, Aiken, SC
'Shichidanka'
'Diadem'
(unknown) - cuttings were obtained several years ago, from the garden of Christopher Lloyd (D) at Great Dixter, near Northiam, East Sussex (UK).
He did not know the name or if it was a named cultivar.

H. x 'Preziosa' is said to be a hybrid between H. serrata & H. macrophylla and I tend to agree, as it is a much taller plant than the serratas. It is the only Hydrangea, among the 150-200 cultivars in my garden, that ever hosted Powdery Mildew! It became so bad that I removed it after about 3 years. Curiously, a rooted clone from that plant, growing only a few feet away, has never been affected, nor were any other Hydrangeas located nearby.

Thank you so much, that's great information. I need to stop searching the web for h. serratas, I'm getting frustrated with the limited choices we have here in the US. Sure would like to see something like h. serrata Autumn Fire in person, though.

Lois in PA

Here is a link that might be useful: Autumn Fire

There is probably a very good reason that cultivars, such as 'Autumn Fire', aren't promoted or readily available.

As the shortened days of fall approaches and the production of chlorophyll begins to decrease, the foliage of the H. serratas, as do the Oakleafs, begin to produce anthocyanins
(Red & blue pigments), to protect the plant from sunlight and delay senescence (leaf drop), so that additional carbohydrates (sugars) can be produced and stored in the plant for use during the winter and foliage production in the spring.
When exposed to sunlight, the H. serrata's foliage will begin to exhibit burgundy hues in the fall, but in order to produces deeper red hues, you would need almost perfect conditions, bright sunlight, warm & dry days and cool, frost-free nights. Cloudy days lessen the need for anthocyanin production and rainfall or heavy dews will leach the chemicals from the leaves.

Soils low in Nitrogen (N) are also necessary for the most brilliant reds, so are soils in the neutral pH range (near 7.0). Soils at the 2 extremes, acidic or basic (alkaline), will produce blue hues, instead of red.

Certain areas of NA, Eastern Canada & the New England states, are noted around the world, for producing the most brilliant red foliage colors in autumn. H. serratas grown there would likely come closer to producing the foliage hues shown by Esveld (NL) for 'Autumn Fire'

Maybe someone from that area could relate their experiences?

Lois__
You are correct regarding the dearth of H. serrata cultivar selections in the US.
Spring Meadow once offered about 15 cultivars in their Japanese Collection and Itsaul Plants has introduced that many and more, but they met with public apathy and most were discontinued. The diminutive Lacecaps and a few very small mopheads, couldn't compete on the sales racks with the huge red, white, blue, pink & purple mopheads of the H. macrophyllas! Only collectors and gardeners with very small gardens or those that grew them in pots on a deck or patio, were customers and that didn't generate enough sales to sustain economical production. So sad!

I'm a big fan of 'Omacha Nishiki', and 'Nachtigall', if you can find them. Great color on both for me.

Wow, Omacha Nishiki has great variegated foliage. But I also found Beni Gaku, which has really splashy looking flowers...

'Preziosa' if grown in considerable sun (4+ hours) produces great fall colors.
Though not being serratas, Lady in Red (burgundy-wine) and Tokyo Delight (orange-red) have very showy foliage under the same lighting conditions.