Sunday, July 26, 2015

Plant Story--Helianthus pumilus, the Dwarf Sunflower

"Ordinary" is a point of view. One person's ordinary is another person's rare and exotic observation.

Helianthus pumilus
Helianthus pumilus, the bush sunflower
I frequently hike along the trails in the foothills of the Rocky Mountains in Larimer County, Colorado. An ordinary plant growing along the trails is a small sunflower, Helianthus pumilus (daisy family, Asteraceae), called the dwarf sunflower or bush sunflower. It is pretty common on the trails and one quickly ceases to notice it. But, it is endemic to this area and you won't see it more than 250 miles north (Casper WY) or 200 miles south (just southwest of Pueblo, CO). Looking at the specimens recorded by the Rocky Mountain Herbarium (link), it doesn't occur  lower than about 5800' or higher than about 8000.' The only place in the world it is an ordinary plant is right here.

The plants are barely knee-high (pumilus means "dwarf") and the ones in Larimer County grow in a rounded, bush-like shape. The other small sunflowers I know spread out, with flowering shoots sticking up like grass stalks, quite a different look.

Helianthus pumilus

As fits a small plant, the flowers are small for a sunflower--but in a good year they can be numerous. They are pollinated by insects, especially small native bees.

Dwarf sunflowers are closely-enough related to the annual sunflower (Helianthus annuus, the cultivated sunflower) that they can cross. The resulting hybrids have been further crossed back to cultivated sunflowers to bring disease resistance and some interesting seed oils into commercial sunflower varieties.

Helianthus pumilus

Recent studies suggest the plant has been in the same--limited--region for thousands of years. Furthermore plants in the northern part of its range are quite genetically different from those in the south, because the Pike's Peak massif creates a zone of unsuitable habitat between the two. That makes a gap that the plants have been unable to cross. They remain compatible--no suggestion of becoming two species--but isolated from each other since at least the last glaciation (21,000 years). Both "north" and "south" dwarf sunflowers grow successfully only in a few of the available foothills environments, with virtually identical requirements.

It is very easy to get complacent about plants if you live in the same place for a while. By that I mean, starting to think that my three local sunflowers are the only ones out there, and, conversely, without realizing it, that everyone else knows these sunflowers. The reality is quite different, though. Most genera of plants have a dozen species, some have 250. There are 70 species of sunflower (Helianthus). If one location has three, in a different habitat or 300 miles away, two of the three species are likely to be different. Look at the sunflower species on the USDA Plants website: link  Lots of different sunflowers! When I look up a plant for this blog, I am repeatedly amazed how many close relatives it has.

Helianthus pumilus
the dwarf sunflower, Helianthus pumilus
As I travel, people often ask me to identify plants. That is so much harder than it sounds. Some few weedy species are common in many places. But those are the exception. Climates and soils change with distance and plants are very sensitive to those changes. Conditions one species of sunflower likes will be unsuitable for another. What species you see change between say, Colorado, Nebraska and Michigan. I may know it is in the sunflower family or maybe even a sunflower of the genus Helianthus, but standing in a park in Missouri, its likely to be a sunflower I don't know.

As always, you can look at that as annoying: "oh no! I have to learn a new plant!"; or wonderful "how cool, a plant I haven't seen before!"

Helianthus pumilus

Similarly, your ordinary plants, overly-familiar to you, will be exotic and romantic to someone for whom them are new.  "Oh! I've always wanted to see a cactus (sugar maple, primrose...) in nature."

Thus if you want to see the dwarf sunflower in nature, walk hiking trails in the foothills on the east side of the Rocky Mountains in northern or central Colorado or similar places in Wyoming. They are
common. But you won't find them anywhere else. Every plant's natural distribution is somewhat limited, but the dwarf sunflower's native range is tiny.
Be sure to enjoy plants that, like dwarf sunflower, are ordinary only in a restricted area.

Footnote: the USDA Plants map shows Helianthus pumilus in Idaho. The Flora of North America says "Colo., Wyo." for its distribution. The herbarium specimens shown online at the Rocky Mountain Herbarium are only from Colorado and Wyoming. Please let me know if you know of a record of it in Idaho and I'll correct this post.

Reeves, P. A. and C. M. Richards. 2014. Effect of a geographic barrier on adaptation in the dwarf sunflower (Helianthus pumilus Nutt.)

Comments and corrections welcome.

Kathy Keeler

1 comment:

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