Some ornamental grasses are sensitive to day length. Emerald Coast Growers.

Day length sensitivity is an important factor to consider when scheduling your fall plantings of ornamental grasses. This can vary in intensity between grass species and can have a huge impact on whether or not your fall plantings finish on time.

Long-day grasses (such as Panicum virgatum) will grow and flower when days are long, and short-day species (like Muhlenbergia capillaris) will flower when days become shorter.

As day length decreases, long day grasses will finish flowering and move toward dormancy, while short day grasses will initiate flowering. It should be noted that it’s actually the length of uninterrupted darkness that truly controls this response in plants, which is why night interruption works. With that said, we’ll still call it day length because everyone else does.

Day-length sensitive grasses

Day-length sensitive grasses like P. virgatum respond strongly to the number of hours of continuous darkness. This can control timing of growth in spring, flowering, and the shift to dormancy which is what you need to pay attention to when trying to finish day length sensitive grasses in fall. How soon they will begin to shut down in fall will vary by the species you are growing and your location. Northern locations can expect to shut down sooner compared to Southern locations, no surprise there.

You can help extend this period by extending day length with supplemental lighting. This can be done by reducing continuous night length by lighting at the end of the day or by providing a night interruption somewhere near the middle of the night. This might not work as well with strongly sensitive grasses such as P. virgatum that want to shift to dormancy even with extended day length, but it might buy you some time.

Grasses that aren’t as sensitive

For species that are not as sensitive, such as Pennisetum sp., extending day length can give you a little leeway when trying to get plants finished before they shut down or even keep them growing through the winter. Keep in mind that grasses like P. virgatum, some Pennisetum sp., and many others will generally overwinter better and come out in spring much stronger when they get a good dormancy. Trying to completely fool them can open the door for problems later on.

At the other end of the spectrum, you have other grasses that don’t really seem to care if the sun is out for eight hours or 14 hours. Some crops don’t have a dormancy photoperiod response to speak of, and probably react to temperature more than anything. Miscanthus is a great example. It will continue to grow until frost or a hard freeze. We often find ourselves waiting for a good freeze to try to get our Miscanthus to go dormant, as late as December outside. Heat is also the way to wake them up in spring if you’re trying to get them moving early.

The bottom line: You want a finished plant with a solid crown and good root mass that shifts its energy stores into its root system as it moves into dormancy. Whether the grass you’re growing does this in September or December, your goal is the same—have well-rooted finished pots. This will improve overwintering success and help plants come out in spring with vigor.

When to pot

Adjusting your potting schedule to account for the different finish times based on dormancy gets rid of a lot of these problems. I’m a big fan of designing problems out of systems. Potting crops up on a schedule that gets them finished at the correct time solves a lot of problems before they start.

One way to control crops with a strong photoperiod response, such as P. virgatum, is by getting them potted early enough in the summer that they will be finished by September or early October. Crops with a weaker photoperiod response such as Pennisetum and Miscanthus can be potted later in summer, as they will continue to grow for an additional four to eight weeks depending on day length and temperature.

If you have potting schedules or other constraints that force you to do all of your potting at once, changing your liner size can also help control finish times. Starting with a larger liner can shorten finish times and get your crops finished on time.

Josiah Raymer is head grower and general manager for Emerald Coast Growers, one of the country’s largest ornamental grass and perennial producers.