The Marijuana scent of autumnal 400-series highways!

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by Fred Schueler & Aleta Karstad – Fragile Inheritance Natural History

Even those who drive with their windows closed may have noticed the distinctive waft of Dyssodia papposa along Hwy 416. This low-growing annual species, with small intensely yellow composite flowers, is afflicted with a variety of English names, including Fetid Dogweed, Fetid Marigold, Prairie Dogweed, and False Dog-fennel. The scent of crushed Dyssodia is sharp and pungent, like pine and skunk – and as fresh and penetrating as menthol or eucalyptus. Dyssodia grows on you: each time you step out of the car on it, you’ll be surprised by its perky pungency, and after several encounters you’ll be pleased that you’ve found it again!

Dyssodia entered our world on 2 October, 2009, when Mike Oldham announced that he was leaving our slug identification workshop in Bishops Mills early, in order to survey it on his way home to Peterborough. Mike is the botanist at the province’s Natural Heritage Information Centre, and he’s famous for noticing interesting plants growing in neglected habitats. In 2009 he’d been documenting the spread of Dyssodia along the superhighway roadsides of Saskatchewan, Manitoba, Ontario, and Quebec. As he went home along the 401, he collected specimens of this abundant, but previously unreported, species in every county along his route.

Dyssodia papposa along Hwy 416 near Kemptville, 16 September 2020.
In some years and areas this narrow band would be expanded into a metre-wide stand. Here it’s growing outside late-blooming White Sweet-clover (Melilotus albus).

Dyssodia is native to the western and south-central Great Plains of the US, but has scattered introductions east into New England, mostly on the shoulders of highways. Its light fluffy seeds make it a prime candidate to be whirled along roads, or to be carried long distances on vehicles. Since Mike Oldham told us about it in 2009, we’ve found it across Ontario along the entire length of the 401, and have seen it in each of the prairie provinces, where it is also considered non-native. Locally, we seem to be near the northern limit of the range: the populations along the 416 generally drop out south of the 417, and we have only a few records from Hwy 417 through Kanata, while east of Ontario it doesn’t even get as far into Quebec as Montreal.

As soon as you know about it, you can notice the narrow carpet or border of this species along the edge of the pavement of the 416 or 401. In September, it’s yellow and green, turning brownish this year while the flowers are still open, and becoming reddish brown in the winter. It’s all along the superhighways, but as you drive off the interchanges, the Dyssodia drops out. It simply isn’t found along 2-lane highways or county or municipal roads.

Since this failure to thrive away from superhighways was so general, and since it’s an invasive species that only invades an artificial habitat, we figured there’d be no harm in seeing how long Dyssodia would persist if it were introduced to an ordinary roadside. So, in November, 2018, we sowed seeds along a few metres of County Road 18 south of Bishops Mills, across from a driveway and above a culvert, where the Counties’ recent harrowing had made the gravel shoulder a full metre wide.

In August, 2019, the seeds at the driveway site had produced a dense 2 m long, 8 cm tall, stand 40-70 cm back from the pavement, and the culvert sowing had produced a very few small plants 90 cm back from the pavement. On 27 August, as flower buds were beginning to show as pale yellow dots, the driveway stand was missed by the Counties’ harrowing of the road shoulder (perhaps because the blade had been raised in areas near houses) and the few plants at the culvert were also missed by the harrowing, with the survivors at the edge of the vegetation among Common Ragweed and Foxtail Grass.

On 6 September, Dyssodia flowers were fully open in the driveway stand, which we hedged with flagging tape to show the adjacent landowner not to mow it, but on 19 September the Counties’ mower bar cut reduced the flower heads by about half, cutting up the marking tape we had around the stand. This mowing reduced the culvert stand to two small plants along the edge of gravel shoulder of the road. At the driveway stand on 13 October, most seeds were shed, and at the culvert stand there were at least two small brown plants that had shed their seeds.

On 8 May, 2020, the plants of the driveway stand were no longer evident, swept or scrubbed by a Counties harrow for a width of 80 cm back from the 25 cm sand shoulder, and it was 30 June before we recognized 4-5 cm seedlings there, among a much denser growth of Matricaria matricarioides (Pineappleweed). On 15 July the still-small plants were driven over by the wide wheels of a trailer taking a house south through the village, and on 28 July they were mowed over by the Counties’ tractor, but without much disturbing the low plants. On 23 August the outlying plants of the driveway stand were 16 m apart, about 50 cm back from the pavement, growing mixed with Green Foxtail grass, though the dense growth was only about 3 m long. The first few blooms opened on 2 Sep- tember, a day when the driveway stand was mostly mowed by a Counties tractor, but the patch of the biggest plants was spared, apparently by the mower bar’s interfacing with a pile of brush left from some clearing of adjacent land. The mowing reduced the culvert stand to one small plant in bloom.

We will see how things go in future years, but for now we conclude that roadside mowing and harrowing oppressed the two road-shoulder stands of Dyssodia that we tried to establish in Bishops Mills, and that this more intense maintenance along county roads may be one of the reasons Dyssodia is only found on superhighway shoulders. Whatever the reason, this restriction is especially striking when you pull off Highway 43 onto the northbound entry ramp of the 416, where the puffy shoulder-edge stand of Dyssodia begins in the first few metres.

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