Even if you’ve already had a frost or two (I certainly have!), it’s probably not too late to save your purple fountain grass (Pennisetum x advena ‘Rubrum’, formerly P. setaceum ‘Rubrum’) from the cold.
This beautiful grass, with reddish-purple foliage and foxtail-shaped flower spikes that dance in the slightest breeze, is a very popular summer garden and container plant. However, nurseries offer it as an annual, so most gardeners simply leave it to freeze in the fall, assuming it’s doomed to die anyway. But it is not in fact an annual, but a tender perennial grass (zone 8 or 9) you can easily overwinter indoors.
And, as mentioned above, it is probably not too late to save your plant, as it will tolerate quite a bit of frost: down to about 15˚F (-10˚C). So if you act quickly, before the real winter cold sets in, you ought to be able to bring it indoors while it is still in good shape.
Easy to Overwinter
Container-grown purple fountain grasses are simple to bring indoors: just clean off the pot and carry them in. If yours are planted in the garden, you’ll have to dig them up and put them in a pot.
Like most ornamental grasses, purple fountain grass does best when you cut it back drastically to remove old leaves. Do this when you bring it indoors, cutting back both flowers and leaves to about 2 or 3 inches (3-5 cm) high. Place the plant near a sunny window in a heated room. Normal room temperatures are fine as are cooler rooms with night temperatures down to about 40˚F (5˚C).
Water the plant like any houseplant, when the potting mix is dry to the touch. Don’t fertilize it for now, though: you don’t want to encourage rapid growth while the sun is so weak: that will only give floppy, etiolated leaves. So put the fertilizer away and let slow and steady be your motto, at least until the longer days of spring.
Note that though the foliage will grow back fairly quickly after the plant’s severe pruning, its new leaves will probably entirely green or only slightly reddish: that’s because the plant’s purple coloration only truly comes out when the plant is exposed to intense sunlight, as in outdoors. Indoors, your plant will look more like a pot of ordinary lawn grass than the splendid purple fountain grass it becomes in summer!
In March, divide the clump and repot the divisions. This will give you more plants to put outside for the summer!
When there is no longer any danger of frost, gradually acclimate the divisions to outdoor conditions and pretty soon you’ll have striking purplish red grasses with swaying flower spikes to brighten your garden all summer!
Do note that you can’t grow purple fountain grass from seed: it’s a sterile hybrid, P. x advena ‘Rubrum’, resulting from a cross between African fountain grass (P. setaceum) and P. macrostachya ‘Burgundy Giant’. That’s good news for Southern gardeners (zones 8 and above), as the species P. setaceum is highly invasive and growing it is illegal in many areas. Since it is sterile, however, purple fountain grass can be grown in tropical to subtropical climates without fear that it will escape and become a menace.
Other Tender Pennisetums
Purple fountain grass has given rise to a few cultivars with variegated foliage, like P. advena x ‘Fireworks’ (red, pink and white), P. advena x ‘Cherry Sparkler’ (pink, white and green) and P. x advena ‘Skyrocket’ (green and white). All require the same winter treatment as purple fountain grass.
There is also a group of pennisetums with purple foliage but that share a different background. Derived from crosses with Napier grass (Pennisetum purpureum), they are grown as perennial grasses in zones 9 to 12 and as annual grasses in temperate areas. They are popular for their dense clumps of fairly broad, deep purple leaves. Napiter grass hybrids don’t bloom in short season climates (actually, they rarely flower even in the tropics). This group includes such cultivars as ‘Prince’, ‘Princess Grace’, and First Knight™. Like purple fountain grasses, since these varieties are not true annuals, they can be kept indoors over the winter. Just treat them the same way you would purple fountain grass.
On the contrary, another ornamental pennisetum, pear millet (P. glaucum), which has given several cultivars with colorful leaves, like ‘Purple Majesty’, ‘Purple Baron’, ‘Jade Princess’ and ‘Purple Jester’, usually is grown an annual, even in the tropics. It’s normally multiplied from seed that you then resow the following spring. If you have one of these in your garden, you can then collect its seeds now… if the birds haven’t already eaten them all!
Hardy Fountain Grasses
There are hardy fountain grasses, too, Chinese fountain grass (P. alopecuroides) being the species most often grown. It has provided many worthy cultivars (‘Hameln’, ‘Moudry’, ‘Foxtrot’, ‘Little Bunny’, etc.), all with the attractive arching foxtail flower spikes that give purple fountain grasses such grace. Chinese fountain grass cultivars are hardy to USDA zone 5 (AgCan zone 6) and will grow in even colder zones with a little winter protection.
Oriental fountain grass (P. orientale) is just a lovely and about as hardy, with pinkish foxtail spikes. ‘Karley Rose’ is the best known cultivar.
Hardy fountain grasses don’t need to be brought indoors over the winter.