Pulling out Arizona Flora (Kearney and Peebles), I turn to Polygonaceae in page 228. I am already sure this is an Eriogonum, but am curious how the Polygonaceae key takes me there. The key question is whether the flowers are subtended by an involucre composed of more or less united bracts. The inflorescense is either a globose unit, or a umbel like group of globose units (actually a Cyme). There is an involucre (though not united) below the inflorescence, and each flower also has its own involucre, though once again not united.
But I answer yes to this question. The next question is whether the lobes of the involucre are spiny at the tip, mine are not, so I am at Eriogonum.
Arizona has 57 species of Eriogonum. The first question in the key is whether the plant is perennial with a woody caudex. My plant is a small shrub with many woody stems, so I go to (29) in the key, skipping perhaps 1/3 of the species right away! The next question is whether the plant has stems that are noticeably woody above the ground, and mine definitely does, so I am on to (30) with the candidates much reduced.
The next question is whether the perianth is silky-villous. The petals are not, but the calyx certainly is. So let's look at the rest of the question. Is the color white or pink, yes. Cymes dense, sub-capitate; leaves pilose or villous above, lanate beneath, revolute. Indeed the leaves below are white and covered with dense white hairs. The leaves roll back on the sides, the top is green with a few hairs.
This appears to be E. fasciculatum, which is very common along the Mt. Lemmon highway above Molino basin. If this identification is correct, it seems surprising to find it at this low elevation in the saguaro forest. However, Arizona Flora gives a range from 1000 to 4500 feet elevation, so it would seem that the plants along the Mt. Lemmon highway are near the top of the range!
There are two varieties. var. polifolium may be the most widespread, but var. flavoviride is said to be found in the Sonoran desert region and has light green leaves that are hairless on top. My plant then seems to be E. fasciculatum var. polifolium.
E. wrightii is also fairly common, but the inflorescence is more open, less globose, and not hairy. It is found from 3000 to 7000 feet in elevation across the entire state. It tends to bloom later (starting in June).
As I look through other entries in the key, I see questions about the achene being winged and how long it is. Fortunately I did not have to answer any of these questions. I am not sure if they would have had me dissect a flower (which I certainly don't mind) or to have mature seed (which I did not find on a second visit to the plant).
In conclusion, this plant immediately reminded me of the E. fasciculatum I had seen along the Mt. Lemmon highway, but I did not believe it until I had closely examined the plant and worked through the keys.
Tom's Plant pages / email@example.com