Harvest time in the wiregrass fields

When should you collect wiregrass seed?

After I typed that phrase, my brain was bombarded by a dozen possible keywords: “location!” “rainfall!” “burn date!” “soil!” “time since fire!” 

If you haven’t realized by now, wiregrass seeds are complicated. Luckily, folks have been collecting wiregrass seeds for a long time, and their knowledge about seed ecology can give us a ‘best guess’ strategy. 

But before we talk about how these collectors tell when wiregrass seeds are “ready,” I think it’s important to clarify the goal of collecting seeds at the proper time. People want high-quality seeds. Wiregrass seed quality can be described by two separate concepts: the amount of filled seed and the viability of the filled seed. Dr. Hector Perez (University of Florida) points out that seed viability depends first on seeds being filled because if the seeds are empty, there is nothing to germinate. 

Wiregrass could, for example, have very few filled seeds, but all could be viable. Or it could have many filled seeds and few could be viable. But an unfilled seed is definitely not viable!

How do I determine if seeds have had their fill?

Before seed harvest, managing wiregrass populations to maximize seed fill is a tricky issue. In her thesis research, Sarah Tevlin (2023) found differences in seed fill between wiregrass populations growing in wetter areas versus those growing in drier areas, and seed fill also varied between populations, even when they were growing in similar soils and had been burned at the same time. The take home? Estimating the amount of filled seeds you’re likely to get is tricky and depends on things like the timing of fire, the soils on which the wiregrass is growing, and precipitation during seed development.

The timing of seed collection likely only affects seed fill in terms of whether the seeds have fully formed and matured. You don’t want to collect the seeds too early before they have finished forming.

We asked Nancy Bissett, a botanist with The Natives, Inc., what her group looks for when determining if wiregrass seeds are ready to harvest. She replied that they used what they call the “snap test.” This is similar to what we described in a previous blog post as the press test.

“If a spikelet is filled with a whole wiregrass seed, it can be tested by breaking it in half, and if it snaps, it contains a whole wiregrass seed that is ready for harvest. You can do this in the field on the palm of your hand or carry the spikelets back to the lab, spread them out on a hard surface, and press down halfway up the spikelet with your fingernail. If it breaks with a snap, it contains a whole seed. Look at the seed under a dissecting microscope to get a better feel for the condition of the spikelets. If it contains liquid or is not quite firmly solid, the seed is not yet ready.”

Another common method of determining whether seeds are ready to harvest is to visit the field in the afternoon when temperatures are highest and relative humidity is lowest. Pinch the flowering head (inflorescence) between your fingertips and run your fingers up the inflorescence. If most seeds come off easily, they should be ready for collection (see video below). Visibly, such inflorescences will have spikelets with bent, rather than straight and vertical, awns (see photos to right). It's important to go out several days in a row because rain or moisture can cause the awns to stay straight even if the seed is ready. You want to check on as many dry days as possible. Additionally, not all plants reach seed maturation at the same time, so it’s important to test more than one plant at a site. In fact, wetter and drier areas within the same area may vary slightly.

To determine if wiregrass seeds are ready to be harvested, run hand up culm. If most seeds come off in your hand, they are ready to harvest.

So that’s seed fill. What about seed viability?

Studies generally support higher viability of seeds from wiregrass populations burned in the late spring/early summer compared to in the winter. For example, van Eerden (1997) found higher germination of seeds from North Carolina populations burned in July as compared to December or January. Sarah Tevlin (2023) also found germination was variable depending on how long seeds had been stored and the temperatures to which seeds were exposed. Both studies, again, confirmed wide variability in germination among wiregrass populations, even those burned around the same time. However, these studies did not necessarily control for possible differences in seed fill.

Evidence suggests that the timing of collection does affect the potential for seeds to germinate. For his 1997 thesis, Brian van Eerden collected seeds from wet sites burned in July and from xeric sites burned in May in North Carolina. When seed was collected in December versus November, the average germination was 28.4% versus 6.8% in wet sites and 12.7% versus 6.5% in xeric sites. There was a lot of variability in germination between plants, however, and even between sites burned at the same time. By December, few spikelets were still attached to culms. The takeaway is (you guessed it!) that there is a high amount of variability in seed viability. Still, it’s possible that a difference of a few weeks could result in a difference in germination capability. This could be because seeds harvested later had higher levels of seed maturity. That said, if you harvest too late, you may find many seeds have already fallen off of plants.

One could test germination by collecting seeds and germinating them at home on filter paper because wiregrass doesn’t require pretreatment and will germinate well at room temperature. Why not germinate in a pot instead? Well, you need to ensure that both the root and shoot emerge from the seeds. Wiregrass is known for having abnormal germination where only the shoot emerges. Of course, a plant without roots won’t survive long. Another way to see if seeds are viable is to do a tetrazolium test. Tetrazolium is a staining liquid; if a seed, cut in half, stains pink, it will likely germinate because the tissue is alive.

So roughly, when should you begin looking?

In our experience, wiregrass seeds begin to mature roughly six months after a fire. Plants burned in May or June should have mature seeds around November, and those burned in January and February should have mature seeds around July. Keep in mind, however, that plants burned in the winter months, like December and January, do not typically produce much seed. Nancy Bissett also pointed out that in their experience, "sites that received heavy rains and flooding since the burn will likely not produce any seed, that happened to us more than once, and we had no harvest." Keeping track of the conditions at your site will help you gauge when to start checking for mature seeds. Jeff Glitzenstein, a private contractor and botanist with extensive field experience, advised that: “if your sites were burned earlier in the growing season (April/May), I might be looking to collect late November through mid-December. If burned later in the season, waiting till late December-early January might be best.”

Don’t wait too long because you don’t want all of the seeds to fall off! David Printiss, former manager at Apalachicola Bluffs and Ravines Preserve, advised that if the goal is industrial-scale seed collection (on the order of thousands of pounds of seed), start collecting early. About a week or two before you expect the seed to mature, begin going out every day at the same time in the afternoon and run your hands up the culms. Once you begin getting a few seeds coming off easily, start collecting so that you're able to harvest from all areas before too much seed has fallen.

How to determine if it’s worth collecting seeds at a site?

Start by using the press test to estimate the proportion of filled seeds you have. This will give you an idea of how many seeds might even have the possibility of germinating. Then, you can germinate the filled seeds and estimate roughly how many are viable. Remember, this is tricky because germination depends on several factors. If you are doing germination tests, keep in mind that this might take a couple of weeks, and you don't want to miss the collection window.

You can do your own press or snap tests or send a sample of wiregrass seeds to the USDA Forest Service National Seed Laboratory in Macon, GA, to determine the percentage of filled seeds. You can also conduct your own germination tests or ask the Seed Laboratory to do germination testing in a controlled environment. Estimating the percentage of filled and viable seeds should help you decide whether collecting at a site is worth your time.

References

Baruzzi, C., N. Medina-Irizarry, M.N. Armstrong, and R.M. Crandall. 2022. Fire season and canopy cover interact to affect reproduction of a pyrogenic bunchgrass, Aristida beyrichiana. Plant Ecology, DOI: 10.1007/s11258-021-01204-7.

Tevlin, S. 2023. Untangling seed quality, germination, and stress tolerance in Aristida beyrichiana to improve the efficacy of resource use in seed-based restoration. M.S. Thesis, University of Florida, Gainesville, Florida.

van Eerden, B.P. 1997. Studies on the reproductive biology of wiregrass (Aristida stricta Michaux) in the Carolina sandhills. MS thesis, University of Georgia, Athens, GA.