Plants of Southern California: Sporobolus species of the Desert side of the Peninsular Range
This page shows how to distinguish the four Sporobolus species found on the desert side of the Peninsular Range, listed in Table 1.
Table 1. The four Sporobolus species discussed here.
| Scientific Name | Common Name | #Vouchers | #iNat |
|---|---|---|---|
| S. airoides | alkali sacaton | 65 | 53 |
| S. contractus | spike dropseed | 17 | 16 |
| S. cryptandrus | sand dropseed | 52 | 11 |
| S. flexuosus | mesa dropseed | 11 | 8 |
Sporobolus plants in our area are strong monsoonal responders, and can be very abundant along roadsides. But in the winter and spring when most botanists are out they have faded from their peak, and hence are not very noticeable. As a result, they are not known as well as they would be if they were spring bloomers.
I noticed abundant Sporobolus plants lining SR74 from the PCT down to the Cactus Spring Trailhead for years before I finally made the effort to figure out who they were. This page is the result of fieldwork mostly in September and October 2023.
It was very gratifying to study our four species since it turned out they can be distinguished at a glance. The Jepson eFlora key uses fairly obscure characteristics of the lowermost inflorescence branches to separate them, possibly because it had to distinguish more species in its key, and possibly because some of these "at a glance" characteristics are not always visible in herbarium specimens, or at different stages of the plant's inflorescence growth.
Sporobolus species are fairly easy to recognize. Their most distinctive feature are the long hairs at the top of the leaf sheaths; see the pix in Fig. 3 below. Other characteristics are an inflorescence that is a panicle with many branches, from which the seeds readily drop from the plant, giving them one of their common names of "drop seed". The scientific name is from the Greek, and means "to throw seed". Our plants are all perennials, with dead inflorescences generally present from previous years.
Most of the time, the mature inflorescence shape is by far the easiest way to distinguish these species, and even a long-finished inflorescence retains its shape well enough to still be useful.
Fig. 1 shows a mature inflorescence from each of our four species, all to the same scale.
S. airoides, infl 20 cm tall x 17.5 cm wide. | 
S. cryptandrus, infl 19 cm tall x 6 cm wide |
S. flexuosus, infl 33 cm tall x 12 cm wide | 
S. contractus, infl 37 cm tall x 0.4 cm wide |
| Fig. 1. Photos of the inflorescences of four Sporobolus species, all to the same scale. Click on the pictures for larger versions that are not to the same scale. | |
Fig. 2 shows how different the shape of the inflorescence is for each species.
Fig. 2. The shape of the infl is outlined for each species, with the outline connecting the tips of the inflorescence branches on each side of the central inflorescence axis. These drawings are not to the same scale.
With mature inflorescences, it is easy to distinguish these species at a glance:
- The inflorescence of S. contractus is very similar to that of deer grass, Muhlenbergia rigens, very long and extremely narrow, and can easily be mistaken for deer grass if you just glance at the plants and haven't seen a deer grass for a long time. Deer grass is generally a giant plant compared to S. contractus, with zillions of stems and leaves, and an infl perhaps twice as long as the one of S. contractus.
- The inflorescence of S. airoides is pyramidal in shape, almost as wide as it is tall.
- The inflorescence of S. flexuosus is very narrowly lanceolate in outline in its topmost 2/3, and its lower part is wider, almost the same width throughout, with something like 30 inflorescence branches.
- The inflorescence of S. cryptandrus is narrowly lanceolate in outline throughout, with something like 20 or fewer inflorescence branches.
Young plants of S. cryptandrus are distinctive since they are very reddish. Their inflorescence starts out very narrow, at which point it looks a bit like the inflorescence of S. contractus. But the branches quickly emerge, giving away the id. See photo of a young plant with six inflorescences at various stages.
Plants of S. flexuosus are very distinctive since their inflorescences are so long, with so many branches, and so flexible, that they get all tangled up. We had to do some work to disentangle the inflorescences of the linked plant to get the photo in Fig. 1.
Fig. 3 shows the leaf collar hairs for each of these species, which don't easily distinguish the species except that the hairs are sparser and can be a bit longer for S. airoides. But they are a great field mark to help confirm you are looking at a Sporobolus. Not every leaf collar will have hairs, so look at a handful of nodes before concluding you are not looking at a Sporobolus.
S. airoides photo by Daniel Donovan. | 
S. cryptandrus |
S. flexuosus | 
S. contractus |
| Fig. 3. Photos of the leaf collar hairs of four Sporobolus species, not necessarily to the same scale. Click on the pictures for larger versions. | |
I thank Don Rideout for help with a lot of the fieldwork.