By Bruce Rottink, Volunteer Nature Guide & Retired Research Forester
Western trillium (Trillium ovatum) flowers are a major attraction at Tryon Creek State Natural Area (TCSNA). I’ve already written three Naturalist Notes about this plant. In the process I’ve accumulated a host of materials that didn’t fit too well with any of those previous notes; the time has come to share them.
How big do trilliums grow?
From May 4 through May 6, 2016, I conducted a survey of trilliums that were growing more than 10 feet from any trail. The areas I surveyed were near the Equestrian and North Horse Loop Trails in the northern part of the park, Center, Big Fir and Old Main Trails in the central part of the park, and Iron Mountain Trail in the southern part of the park. For each triple-leafed trillium I encountered I measured the distance from the ground to the attachment point of the triple “leaves.” I also noted whether or not each plant was flowering. The results are shown below.
Ten inches is about the height where the plants shift from non-flowering to flowering, although a couple of very short plants flowered, and a few fairly tall ones didn’t.
What happens to the trillium’s flowers and seed pods?
Ideally, the flowers are pollinated, the seed pods (or capsules) mature, then open and release their seeds into the forest. Of course there are other possibilities. Deer, it has been reported, sometimes eat the flowers. The contents of the maturing seed pod are very nutritious, and researchers have reported that both deer and mice sometimes eat the seed pods.
To assess this, I conducted surveys in two different years. The first survey was conducted between June 23 and 28, 2015. Two groups of trilliums were surveyed. “Trailside trilliums” were those growing within 10 feet of the trail. “Mid-forest trilliums” were those growing more than 10 feet from the trail. It should be noted that at this time the seed pods are well along the path to maturity. The plants were placed in three categories: a) capsule intact, b) pedicel only, meaning the plant had flowered, but the flower/seed capsule was missing, and c) did not flower, as indicated by having no pedicel or capsule. Illustrations of each class are below:
The results are shown below:
A statistical analysis (Note to nerds: I used the Chi-squared test.) clearly shows that a significantly higher percentage of the trailside plants flowered compared to plants growing more than 10 feet from the trail. The cause of this difference cannot be determined by this study. The other statistically significant difference between these two groups is that a higher percentage of the “mid-forest” trilliums only had a pedicel (“flower stalk”), which means that either the flower or the seedpod was removed. Animals likely ate these seed pods or flowers. Perhaps the deer are more comfortable eating in the middle of the forest than trailside.
In 2016 I undertook a second trillium survey, this one was conducted May 4 through May 6, at a time when the last of the trillium petals had just recently fallen off the plants. This time, however, the “trailside plants” I tallied were within 3 feet of the trail. The “off-trail plants” were growing more than 10 feet from a trail. At this time the seed capsules were small and immature. The results are shown below:
Once again, the percentage of plants flowering was statistically significantly higher in the trailside plants compared to plants growing “off-trail”. A slightly smaller percentage of the flowers/seed-capsules had been eaten than in the previous study, probably because there was a shorter period of time for the animals to eat them, or perhaps, being smaller, they were of less interest to the animals. The one thing that clearly stood out in the data is that the percentage of reproductive structures missing was significantly higher along Old Main Trail than along the other trails. (Geek note: Statistically, there is less than a 1 in 10,000 probability that the higher percentage of missing capsules observed along Old Main was due to chance.) The fact that Old Main is one of the most heavily traveled trails makes it tempting to speculate that people were picking these flowers, as shown in the picture below, but this study cannot prove that.
An alternative explanation for the empty pedicels is that the flower was defective and the defective bloom was aborted. I recently saw a single dysfunctional bloom in the forest. It appears that the plant started to produce a functional flower, but something bad happened along the way, as in the example below, where you have what appears to be an attempt at a flower, but no actual petals.
Does it really take 7 years for a trillium to recover after the flower is picked?
Many people believe that if you pick a trillium it will be 7 years before the plant flowers again. I unexpectedly got a chance to test that theory when someone picked 6 trillium flowers on a plot of trilliums I had been studying for 5 years. I concluded these had been picked, because the remaining stems did not exhibit the type of cut associated with animal browsing. One of the stems from which the flower had been picked is shown below:
Although very unhappy, I decided to capitalize on the tragedy. (“When life hands you a lemon, make lemonade!”) I carefully documented the exact location of the trilliums. Based on the location of the six flowers, it appeared that all of them were twin stems, arising from a total of only three rhizomes (rhizomes are like a flower bulb).
In April 2017, one year after the tragedy, I surveyed the site again. I went to the site and measured the location of the trilliums. Based on their location, 2 flowering stems were within ½” of the location of the flowering stems that were decapitated last year. Thus I concluded that only 2 out of the 3 effected rhizomes produced flowering stalks again this year. However, whereas last year they were twin-stalked, this year they only had one stalk each. At the location of the third picked trillium, there was nothing. Scientists have determined that some years a trillium will occasionally just take a rest, and not produce an above ground stem. So that is what this one did, OR it died. I don’t know which. Either way, for at least two of the plants, the “7 year” myth is debunked.
Does anything eat trilliums?
Deer are commonly reported to eat trilliums. But it turns out that isn’t the whole story. This spring I ran across one of our slimy forest friends deeply engrossed with a trillium. Note that this is not our native banana slug (Ariolimax columbianus) but appears to be one of the non-native species.
You’ll notice that considerable chunks of the trillium petals are also missing, and these may only have been the prelude to the slug’s full scale attack on the heart of the trillium flower.
What pollinates trilliums?
Most of the plants we hear about are pollinated by bees, like our Pacific waterleaf (Hydrophyllum tenuipes), or by the wind, like Douglas-fir (Pseudotsuga menziesii). Trilliums are a little different. A study of Trillium ovatum in southern Oregon determined that pollinators included several species of beetles, honey bees, bumble bees, crab spiders and geometrid moths1. Since the trillium doesn’t produce nectar, at least some of these creatures are here to eat the pollen, and they spread the pollen as an unintended side effect.
The forest is endlessly fascinating, when a person just stops to observe. Looking back on my old trillium photos, I now see lots of the “little brown bugs” deep down in the bloom. How could I have missed that so often? When you’re out in our forest, stop for a minute and look around. I think you’ll be amazed, as I was, at how many interesting things are out there.
1Jules, Erik S. and Beverly J. Rathcke. 1999. Mechanisms of Reduced Trillium Recruitment along Edges of Old-Growth Forest Fragments. Conservation Biology 13:784-793.