Spacing is a matter of what you want to achieve. Specimen means to have it standing alone, or near something entirely different. Mass means close together for effect. For a mass of them, I'd go as close as 18".

Size depends entirely on how the weather goes. 24" is pretty small if it's really P.a. 'Rubrum.' It might be right for one of the dwarf cultivars, but 'Rubrum' can hit 5' if it's happy.

Wow. Thanks for the advice, donn. I'm assuming you're talking about within one season, since you're 7a. I had no idea. Even if it's something like 3', that majorly changes my plans, since my garden is relatively small.

The tag says P. rubrum "Summer Jazz". I've done some research but I've been unable to come up with the species name, that is, whether or not it's P. alopecuroides for sure. It does look like the usual purple feather grass.

'Rubrum' is a Pennisetum x advena. The 5' is undoubtedly where it's hardy, but I'd think you'd still get more than 2' out of it, if the weather's good throughout the season.

Whoops, guess I just made up that species name. Thanks for the correction. I was thinking of doing a massed "wall" thing. I'll try 2' and hope for the best.

I'm going to really have to get used to referring to 'Rubrum' as an advena rather than a setaceum.

I;m curious, was there a tag description for 'Summer Jazz'?

Here's what San Marcos Growers has to say:

"For many years most nurseries and references have listed this plant as a variety of Pennisetum setaceum, either 'Rubrum' or 'Cupreum'. While it superficially resembles Pennisetum setaceum, a declared noxious weed in the western US, this plant has been described by some grass taxonomists as actually being P. macrostachys and we listed it this way for several years but the name was confusing to many people and we reverted to the admitably incorrect name that most people were familiar with. Recent work in preparation for the grass sections of the Flora of North America, which will include naturalized and cultivated grasses, indicates that the name chosen for this plant will be Pennisetum advena or perhaps P. x advena. Dr. Joseph K. Wipff, previously with Texas A&M and now a turfgrass breeder, wrote the section on Pennisetum and has indicated that Red Fountain Grass is most likely a cross between P. setaceum and P. macrostachys (AKA 'Burgundy Giant')."

The whole Wipff/Pennisetum story is here, but you'd better have a botanical dictionary handy.

Yikes. No gg, there was no tag description. Interestingly, I was paging through a White Flower Farm catalog this morning and happened on their photo of P. setaceum. I'm pretty sure it's my plant. Their recommended spacing? 15 inches (maybe that works in Connecticut, land of short summers?) They do usually test their stock in their trial gardens.

donn, thanks for the reference and link. The tag lists rubrum as a species name rather than a variety, hence my confusion.

For my zone the 15 inch spacing that WWF suggests is quite adequate for an annual season. I use a lot of the red fountain grass and usually try to buy the bigger ones (not the biggest ones available, but not little starter plants) I can find so that I can get some impact in a short Michigan growing season. Buying the little 3 inch pots with a few stems in them is pretty useless around here, even if they quadruple themselves & more, it's still not enough plant to make it worthwhile.