Problem-free Panicum

Panicum virgatum and its cultivars are popular, long-time favorites. They combine beauty, toughness, and adaptability with a winning combination of ecological value. Most of our nursery and greenhouse customers are growing several varieties, and they know they can come to us for assistance. The most common production concerns we hear about are yellowing, rust, and slow emergence in spring. Getting a handle on Switchgrass is important for any grass program, so we wanted to share our experience.  

We focus first on understanding the range and warm-season nature of Switchgrass. Then we address cultural conditions and management practices, which are often related to yellowing and rust problems. As warm-season grasses, Switchgrasses are some of the last to emerge in spring, so we'll cover production scheduling and finish timing to help you take advantage of the Panicum growth cycle.

Understanding Switchgrass

Switchgrass Snapshot

Panicum virgatum is the most widely grown species in the genus. Commonly called Switchgrass, it’s a long-lived perennial native to North America. It is deeply rooted, somewhat rhizomatous, and sensitive to photoperiod.

Panicum virgatum has a wide native range, from southern Canada down the U.S. to Mexico, Cuba, Bermuda, and Costa Rica. It was a major component of the tallgrass prairie that used to be prevalent in the central U.S. It’s also found on dry slopes, sand, open oak or pine woodlands, the shores of lakes and riverbanks, and in brackish marshes. It can grow and thrive in many climates, soil types, and land conditions. The growing season in northern habitats can be as short as three months; in southern reaches, up to eight months. In addition to supplying forage for livestock, Switchgrass provides food, cover, and nesting material for wildlife.

A Wide-ranging, Variable Species

With such a large geographic and cultural range, its form and characteristics vary widely. Botanists have identified two broad ecotypes—upland and lowland—which reflect some of those differences. Upland types tend to be from more northern latitudes and exhibit greater cold tolerance. They tend grow in drier soil conditions and tolerant drought better, while being smaller and shorter than lowland types.

The lowland types tend to be from more southern latitudes and found in moister conditions. They grow well with plentiful water and grow larger than lowland types. They often have blue-green waxy leaves, a later flowering time, and seem more resistant to rust diseases. Many of the observed physical differences in the two types have to do with differences in water use and photosynthetic rate.

While the two ecotypes show different characteristics, there are also large variations within the two types that can overshadow the broader ecotype differences. That makes sorting out cultivated selections challenging. Forage-type cultivars are developed based on regional ecotypes, but horticultural selections can have a mix of ecotype characters. It’s not clear where common cultivars in the trade fit within those ecotypes. This is an area that needs more exploration and study.

Uber Efficiency

Panicum virgatum is a warm season grass, and that’s fundamental to understanding and growing it. Most land-based plant species (85%) fix carbon using a C₃ photosynthetic process. We commonly call them cool season growers. Under environmental pressures, structural and chemical adaptations in grasses and other plants over millennia gave rise to a C₄ photosynthetic process. We refer to these as warm season growers.

Warm season grasses have greater growth potential and greater efficiency in use of water, sunlight, and nitrogen. They grow best when soil temperatures are between 70°–90°F and ambient temperatures in the 80°–95°F range. Compared to cool-season grasses, warm-season grasses can produce the same amount of biomass using one-third to one-half of the water. On one-half the nitrogen, they can photosynthesize just as well. They can also reduce water loss under dry conditions by closing their stomata during the day. That makes them less susceptible to drought stress.

Their C₄ photosynthetic pathway and uber-efficiency play a big role in understanding the growth of Panicum virgatum. It breaks dormancy later than cool season grasses, and the greatest growth occurs in midsummer. That’s when it can take full advantage of heat and sunlight. That pattern is even more obvious in colder regions, where Switchgrass breaks dormancy late in spring and summer heat is slow to arrive. Moreover, copious amounts of nitrogen and water can lead to lush growth and flopping. These are plants that grow quite well under lean conditions.

Cultural Conditions

Get a Good Start

Keeping Panicum problem free starts at the beginning. When transplanting liners or plugs, match the original soil line of the propagule with the media in the container. Grasses are sensitive to having the crown buried, which limits air circulation and can invite disease. Planting too high can leave the root ball vulnerable to drying out.

You’ll also want to make sure there is good root contact with the media and there are no air pockets. Once potted, the root zone should be kept moist, but not wet, until the plants are established.

Manage Water & Nutrients to Prevent Yellowing

Switchgrass grows best in full sun conditions and has average water needs for an ornamental grass. It needs less than Pennisetum but more than Schizachyrium. If you’re growing outside, use a light porous media to facilitate good drainage. Panicum virgatum can take up a lot of water, but that can have a significant downside in production. Frequent, heavy rain events or overwatering can lead to yellowing—one of the major issues we hear about from customers. Keep an eye on your irrigation regimen in the greenhouse and back off if you start to see yellowing.

Use a well-drained soil mix to help with water management. Our current media is 60% pine bark, 30% peat, and 10% perlite. To this basic mix, we add 5 lbs. of dolomitic lime and 1 lb. of Micromax per cubic yard. We also include a 5- to 6-month slow-release fertilizer (SLR). The rate depends on whether we’re potting up for fall/winter production (4 lbs./cu. yd.) or summer production (6 lbs./cu. yd.).

Switchgrasses aren’t heavy feeders, so incorporating SLR in the potting media usually covers a single round of production. However, if we wind up overwintering plants, they get a 1 lb.-rate of SLR just before they break dormancy. Overwatering isn’t the only cause of yellowing in Switchgrass. Nutritional imbalances can send Panicum in the chlorotic direction. Check your EC and pH levels to make sure they’re not way off. If we see yellowing on a crop, a single application of liquid feed 10-10-20 with iron usually turns it around.

Pest Pressures and Management

Compared to other perennials, insect pests are minimal on Panicum. We see spider mites in summer, along with occasional Japanese beetles, thrips, and spittle bugs. The heaviest disease pressure comes from rust (Puccinia and Uromyces). Rust problems tend to be very site-specific, depending on the region and weather conditions. It’s seasonal for us in central North Carolina, usually appearing in September and November.

Our strategy is to spray prophylactically every two weeks starting in mid-August. We use a broad-spectrum fungicide and continue through October or until the plants start going dormant. We rotate chemicals to prevent resistance.

We also see Leaf Spot (Bipolaris oryzae) on Switchgrass. It affects the foliage appearance but does not compromise plant health unless left untreated. In addition, we’ve found the following diseases on occasion, but they are not common: Alternaria, Collectotrichum (anthracnose), brown and root rots, Phyllosticta, and Septoria.

An integrated pest management (IPM) program will keep your Panicum in good shape. Monitor irrigation and limit the time the foliage is wet. Scout regularly and be ready to treat. IPM includes a tolerance for some damage, although we try to keep that to a minimum. Occasionally plants will have residual damage even though there are no active pests present. Work with your vendors and customers to manage expectations and ensure pests aren’t coming in or going out with your Panicum.

Production Scheduling

Typical Growth Cycle

Panicum virgatum is a warm-season grass that grows strongly through the heat and long, sunny days of summer. It is photoperiodic, with flowering connected to the change in day length that comes with the summer solstice. It flowers in mid- to late summer (July and August for us) and will show fall color as temperatures drop and day length shortens. Switchgrass is dormant through the colder winter months and begins growing again in late spring when soil and air temperatures are rising.

In our cold frames, Switchgrasses typically break dormancy around mid-March. They are one of the last groups to emerge, and bud break often varies by cultivar. Moreover, we see variations within crops, particularly with plants on the edges or in colder parts of the enclosure.

Scheduling for the Finish

Production conditions vary across regions, so we’ve synthesized some rules of thumb based on our experience, feedback from customers, and articles by perennial consultant Paul Pilon. Assuming 65° to 70°F throughout crop production, a 32-cell liner should finish in a gallon in 7 to 9 weeks. A 72-cell liner to gallon should take 10 to 12 weeks.

If you’re aiming for 1-gallon sales in late summer or fall, you can pot up in spring or early summer of the same year. Panicum virgatum’s late emergence becomes a plus because you can schedule them for after you’re done with other perennials. They’ll grow quickly once soil and temperatures rise.

When target sales are for spring or for 2 gallons or larger, the best strategy is to pot during summer or early fall of the preceding year and overwinter them. Make sure you pot them up in time to establish roots before they enter dormancy. That head start on root growth will give you a stronger, faster-finishing plant than if you’d potted it in the current season. This timing works for 1-gallon containers and for larger sizes.

Suggested Timing for Liners

Ideally, you’ve got liners ready to go when you need them and on your schedule. To do that with Switchgrass, it’s important to understand timing.

We work with Panicum’s natural growth cycle and only do production rounds certain times of year. For example, we make early spring crops of Switchgrass in late summer of the preceding year. If you’re bringing in liners for spring production, and you’ve reserved that spring crop, you’re all set. But if that spring crop sells out, you’ll have to wait until early summer when the next crop is ready. That can throw off your schedule and add to your costs. The smart move is to reserve them ahead of time.

Here are some guidelines to make sure you’ve got liners when you need them:

Target potting window│reserve liners by

• January through May│reserve by end of previous July
• June through October│reserve by March of same year
• November through December│reserve by July of same year

Another point to keep in mind—if you need a larger quantity or a different cultivar than usual, talk with us even earlier. Like you, we’re planning several years out. Including your needs in our production schedule gives you a leg up and helps keep your production on track.

Bring It All Together

Getting your production timing down and working with their natural growth cycle can help you take of advantage of all that Switchgrasses offer. They can seem tricky, but we hope you’ve found information here that will help you avoid many of the common concerns. 

Remember that they’re very efficient with water, nutrients, and sunlight when temperatures are high. These warm-season grasses have great growth potential and knowing how to manage that is key. Hoffman Nursery is here to help you grow good grass, so give us a call or email if you have questions.

Click here to see a list of references consulted to compile the information on this page.

To read more about Switchgrass (including some cool botanical bits), see our general Panicum page.