The short grass may be Poa secunda (see Moulton). This entry was written in North Dakota along the Missouri River (currently Lake Sakakawea).
Capt. Lewis. April 14, 1805--the bottoms are wide and low, the moister parts containing some timber; the upland is extreemly broken, chonsisting of high gaulded nobs as far as the eye can reach on ether side, and entirely destitute of timber. on these hills many aromatic herbs are seen; resembling in taste, smel and appearance, the sage, hysop, wormwood, southernwood, and two other herbs which are strangers to me; the one resembling the camphor in taste and smell, rising to the hight of 2 or 3 feet; the other about the same size, has a long, narrow, smo[o]th, soft leaf of an agreeable smel and flavor; of this last the A[n]telope is very fond; they feed on it, and perfume the hair of their foreheads and necks with it by rubing against it. the dwarf cedar and juniper is also found in great abundance on the sides of these hills. where the land is level, it is uniformly fertile consisting of a dark loam intermixed with a proportion of fine sand. it is generally covered with a short grass resembling very much the blue grass.
The short grass is probably Poa secunda. The "sweet potato" may be Lomatium cous or Lomatium macrocarpum (see Moulton). This passage was written in Walla Walla County, Washington.
Capt. Lewis, April 30, 1806 --this plain as usual is covered with arromatic shrubs hurbatious plants and a short grass. many of those plants produce those esculent roots which form a principal part of the subsistence of the natives. among others there is one which produces a root somewhat like the sweet pittaitoe.