Noxious Weeds at the Santa Rosa Plateau: Italian Thistle, Carduus pycnocephalus
The following map shows the extent of the Italian thistle, Carduus pycnocephalus invasion at the Santa Rosa Plateau in 2005. Surveys were done multiple times along all of the trails plotted. Italian thistle is easy to identify when it is in bloom and for months afterward, so I am confident that this is a quite complete survey. (The map itself may not report all locations in which I found infestations, since I have not yet thoroughly reviewed all my notes.) No off-trail areas were surveyed, so areas in-between trails may or may not contain infestations.
The map shows the entire public area of the Santa Rosa Plateau Ecological Reserve, with areas containing Italian thistle shown by filled pink circles. No attempt has been made to distinguish the numbers of plants in each infestation. That will be done in the future.
The bold black line is Clinton Keith Road at the north, changing to Tenaja Road, and then Via Volcano Road in the south. Each hiking trail is shown using a solid line in a different color; drainages are all shown as dashed blue lines. The three vernal pools on the Mesa de Colorado are shown as dashed blue-green lines. The Mesa de Colorado is outlined by a faint gray dashed line at lower left. The visitor center is shown at its actual size as a small filled black rectangle just below the first western tributary to Cole Creek.
The map makes it clear that the major infestation is along the southwest arm of Cole Creek, and is probably traveling along that branch of Cole Creek. Nearly all of the other locations are probably due to wind transport off the Mesa de Burro, where Carole Bell reports that there is another major infestation, with possibly some contribution by wind transport off the infestation on the Mesa de Colorado. Exceptions are the patch behind the Visitor Center, which probably was brought in on the mulch used for the planned garden at that location, and the single plant on the Stevenson Canyon Trail (the northwest location).
Every infestation is along or very close to a drainage or an area that has standing water, with two exceptions:
- a group of ~200 plants on the South Los Santos Trail at the lip of the Mesa de Colorado. It is possible this is a wet location, but it wasn't obvious to me at the time. I think it is more likely that a single plant got transported here in a past year, and all of these plants are its seedlings.
- two individual plants along the Wiashal Trail at the extreme north in the plot. These are quite exceptional, since these occur in chaparral, not in grasslands or in major drainages. I think one of these plants is in a wettish location; I'll have to check on the other.
Note that the plants on the Vista Grande Trail, the easternmost locations in the middle of the map, are along a drainage that isn't plotted on the USGS topographic map and which I have not yet added to the map.