If you are in Arizona or just south of the Arizona border, there are two possible bush-mallows to look for out there, maybe more. One of these is only known from collections taken in 1852 and the other is a more recent discovery with few known locations. As with all bush-mallows, relatively recent burn areas are the best place to look. Follow this link for an interactive fire map. See also my post here on what to focus on if you want to be able to ID a bush-mallow from photos.

The type specimens of Malvastrum thurberi which is currently a synonym of Malacothamnus fasciculatus are labelled as coming from Santa Cruz Valley, Sonora, Mexico and were collected in July 1852. Based on what I can find so far, this may mean the location was anywhere between Tuscon, Arizona and Santa Cruz, Mexico. All of the Arizona portion of this area was part of Mexico until the year following the collection.

Below is a scan of a type specimen (larger version here).

The second bush-mallow was collected at Spur Cross Ranch Conservation Area in Maricopa County in 2010 and called Malacothamnus fasciculatus. Based on the photos I've seen, these plants look most like southern populations of what is called Malacothamnus orbiculatus from Los Angeles and San Bernadino Counties in California. Gene Sturla has found additional plants in Tonto National Forest near Sycamore Creek which is also in Maricopa County. These were flowering in September. Below are photos of these plants taken by Gene.

(Update 4/1/19) I found one population. These are growing in a burned juniper woodland. This area burned 7 years ago and ~75% of the population is dead, so new populations are probably unlikely to be found in burn areas much over 10 years old. I would expect them potentially in any burned shrubland or woodland, but they do seem to be pretty rare in AZ. Lower elevation plants have been found flowering in late March and early April. These will likely flower in May. Go find more!
Observation here

The second challenge is to find some living plants near the type locality of horehound bush-mallow (Malacothamnus marrubioides). If you live in the Friant area or want to go on an adventure, here is some information to help you possibly find this species. Any observations of any bush-mallows from this area would be very useful information as the nearest other bush-mallow observations are over 30 miles away. All other specimens that are currently called horehound bush-mallow are over 100 miles away. Being that disjunct makes me wonder if they actually are the same taxon.

The type locality of horehound bush-mallow is supposedly from Fort Miller which is now under Lake Millerton. It appears that a single specimen was collected and a few other specimens have been made of fragments of this specimen. The expedition this was collected on stopped at Fort Miller for a week at the end of July in 1853. I could find little detail about that week, but I suspect the specimen may just be from the general area of the fort and that they likely went exploring. The elevation of the fort is a bit low for at least one of the species that was attributed to there. Here is the exploration record if interested. It’s pretty cool.

Below is a map (larger version here) which shows the presumed locality as a star. The red areas are recent burns and the best likely locations to search for the plants. Follow this link for an interactive fire map.

Below is a scan of the type specimen (larger version here).

This illustration is from the published account of the expedition. Larger version here.

Here are some photos of what are considered horehound bush-mallow north of the Los Angeles area.

See also my post here on what to focus on if you want to be able to ID a bush-mallow from photos.

This is the first of several bush-mallow challenges I’ll be posting for this project. If you live in the Ukiah area or want to go on an adventure, here is some information to help you possibly find this species. Any observations of any bush-mallows from this area would be very useful information.

According to the California Native Plant Society, Mendocino bush-mallow (Malacothamnus mendocinensis) is confirmed from only two historic collections near Ukiah, CA, the last in 1939. It is currently presumed extinct, but bush-mallows are mostly fire-followers and there have been no fires near the type locality for a long time. There are some recent burns relatively nearby which may be worth searching though. The best time to find it in flower would likely be May through July based on the few existing specimens.

Below is a map (larger version here) which shows the presumed localities as a star and an asterisk. The red areas are recent burns and the best likely locations to search for the plants. Follow this link for an interactive fire map.

Below is a scan of one of the type specimens (larger version here).

This crop shows the “conspicuously angulate and striate stem” which is one of the key distinguishing features of this species.

See also my post here on what to focus on if you want to be able to ID a bush-mallow from photos.

A couple of the tricky things about bush-mallows is that the flowers all look pretty much identical from a strait-on view and the leaves can be really variable. This doesn't mean a photograph of them isn't useful, but I'm going to show you what will help most.

Most important, take a close-up side-view image of the flower. The calyx, bracts, and hairs are very important for ID. If the flowers aren't open, the buds can still be very useful. Here's a good example of Nuttall's bush-mallow (Malacothamnus fasciculatus var. nuttallii) showing the side view of a flower and bud:

If the inflorescence is really dense, you can pull off an individual flower and just photograph that from the side. If you do this, try to get the three bracts just below the calyx. The length and shape of these compared to the calyx is very important. Here's and example of a many-flowered bush-mallow (Malacothamnus densiflorus) where I plucked off a flower for a better photo:

It's also good to have a photo of the whole inflorescence. Here's again Nuttall's bush-mallow:

Stem hairs are extremely important. Hairs can vary along the stem based on the maturity of that area. If I only photograph one spot, I usually focus near the base of the inflorescence as that area is more consistent than the various maturities near the tip. Note in this photo how you can easily see the individual stem hairs on the left and not so much on the right:

Glandular hairs are often very difficult to see and photograph, but are very important for differentiating some species/taxa. Note the thicker-based, yellow-green hairs in this photo of Arroyo Seco bush-mallow (Malacothamnus palmeri var. lucianus):

While they may not get you to the species by itself, leaves can certainly help. Color, hairs, shape, and difference between top and bottom can be helpful. Higher/immature leaves may look quite different from lower/mature leaves. Note in this photo of horehound bush-mallow (Malacothamnus marrubioides) I show both sides of the leaves:

Recently I found that the way the flowers dry may be important for identification. Note in this photo of Heller's bush-mallow (Malacothamnus helleri) how the flowers are drying fairly open:

In this photo of arcuate bush-mallow (Malacothamnus arcuatus), you can see how the flowers are drying closed:

If you are in a desert transition area and all you have is fruit, take a photo of the carpels which will distinguish Malacothamnus from Sphaeralcea:

The more photos you take of different parts of the plant, the easier it is to identify. Good luck!