Ariocarpus retusus 'furfuraceus'

Ariocarpus is a genus of 'living rock' - unusual and very desirable cacti. Trademark fall blooming often makes Ariocarpus the last flowering plants in collection.
In culture all species of the genus are not difficult but slow, and large old specimen always attract attention of fellow aficionados.

Echinocereus reichenbachii

Echinocereus reichenbachii is a very variable species, popular with collections due to compact growth and freely produced and nice flowers. The flower in the picture belongs to one of it's forms,  blooming very small and with that coppery shine on 'petals'.

Mammillaria lenta

Mammillaria lenta is an old classic, well known but not very common in collections. It is somewhat slow plant that may give trouble in culture unless well-draining pure mineral soil used. Plant has an appearance of a tight ball of threads, and sporadically produces flashes of nice white flowers.

Lobivia chamaecereus

This plant is known to old-timers like me as Chamaecereus silvestrii, and often was found among the very first plants in new collection, usually a pop received as a gift from kind friend. The plant is very resilient and if grown with plenty of sun light produces its bright flowers with ease. Despite it's soft green look, the plant readily sustains direct sun along with lots of heat, slowing down only at the hottest days.
It usually is considered a spider mite indicator plant, but when grown at bright light it actually not at all suffers from it's attacks. Since moving to CA I've never seen spider mites on that plant.

Notocactus mammulosus

Genus Notocactus is currently merged with older genus Parodia and it probably makes sense, but from the collector perspective it is usually not hard to tell one from another, so I'm using the old name here.
Notocactus mammulosus is somewhat neglected by collectors species, by no good reason. It is easy to grow plant, forming flattened disk-like stem with distinct white areoles and dark stubby central spine. Large silky flowers come only once per season, but they are very noticeable and usually abundant.

Stenocactus vaupelianus

Stenocactus vaupelianus (albatus) is a nice Mexican species, easy and common in cultivation. Pail yellow flowers are not as notable as with other Stenocactus species, but the overall plant appearance is of some sort of cake - dark green stem with wavy ribs, large white areoles and dark contrasty spines. It is easy and common plant, still one of my favorites.

Echinocraus pectinatus

Echinocraus pectinatus is well-known to cactus collectors and variable species from Mexico. Plants vary in spine color, shape and color of flowers and grow habit. 

The pictured plant is a form from Coahuila state, with pink dense spines. Flowers are large but never open fully. 

Mammillaria matudae 'compacticaulis '

The plant in picture I have started from seed about 10 years back, and it developed in modest looking but neat and freely flowering specimen. It is not M. matudae you usually see in cultivation, but a different form, close to what Reppenhagen called M.compacticaulis. This form is somewhat intermediate between M. matudae and M. backenbergiana, both growing in the same general area - southern part of Mexican state Mexico.

Mammillaria sphaerica

Mammillaria sphaerica is one of the most striking Mammillarias, due to its large and showy flowers. This plant is native to Texas and Mexico down to Tamaulipas state. It is moderately frost hardy, and makes no problem in pot culture.

Ariocarpus fissuratus

Ariocarpus fissuratus is the only Ariocarpus native to US. It occurs in Southern Texas Bib Bend area. It is slow but not very difficult plant in culture, most of it is underground thick taproot. The flowering time for all Ariocarpus species is early fall. 

Echinocereus papillosus

This is a brown-spined form of this nice species, a low spreading plant with huge flowers. Plants are easy bloomers here in California, with no special care requirements in my greenhouse. Flowers make them a must have plant for any cactus collection.

Mammillaria dioica

Mammillaria dioica is common plant in Southern California desert and down in Baja California. This picture is taken in Anza Borrego State park near Cactus Trail - a short nature walk near Tamarisk campground, and less than 100 miles from Mexican border.
This Mammillaria comes from the area where summer rains are regular event, and it responds well to the 'common cactus culture'. In culture blooms freely and repeatedly over the summer.

Opuntia engelmannii

This large Opuntia plant was flowering in Organ Pipe NP. I'm not an expert in Opuntias so ID is not guaranteed to be correct.Opuntia engelmannii is one of more common and variable species in the area.
In many places in Arizona cacti are dominant plants, and Opuntias usually the most common. They are robust and very resilient plants, especially in desert area that get relatively large amount of summer rains.

Escobaria vivipara

Escobaria vivipara is a wide spread and variable species. This young plant with neat tight spines grows in Arizona at the slopes of Kaibab plateau. I do not know if this form has any name. The plants in that location are uniformly small, larger plants have more developed central spine. The plant in picture is about the size they start show first buds.

Echinocereus cocinneus

Echinocereus cocinneus in Zion National Park. This is one of the larger plants at that location. Only once I've seen these magnificent clumps in full bloom, all covered with hundreds of scarlet flowers. The intensity of flowering is very condition-dependent.
Several years back these plants suffered from grass fire, started at, ironically, firefighter facility, removed since. The plants that survived recovered well and had overgrown the fire damage.

Opuntia basilaris

Opuntia basilaris has a large range, and shows expected local variability. The plant in picture grows near Wickenburg, AZ. The plants at this locality develop unusually large round pads, and often form large crawling-low bushes of 100 pads and more. You can easily count about 50 flower buds that the plant picture has produced this year.

Cylindropuntia echinocarpha

Cylindropuntia echinocarpha flowering near Joshua Tree NP in California. This plants form low bushes with distinct main stem - trunk, each specimen looking like a spiny tree. In the fall the plants will be shivered so no green seen through the spines, and will stand true to their names Silver" and "Gold" cholla - depending on the spine color. Each spine of  'true' Cylindropuntia has a paper-like sheath, and color of spines is actually the color of that sheath.

Flowers are also nice with their shades of green   

Echinocereus mojavensis

This photograph of Echinocereus mojavensis is taken not far from Utah-Nevada border, close to Grand Basin national park. The cacti here are not that common as in Arizona, but definitely present and not too difficult to find. This volcanic rock outcrop is hosting several large multi-headed Echinocereus mojavensis specimen, along with different kinds of desert brush. In the winter this area sees below freezing temperatures and snow, and summers are more mild than down south.

Mammillaria tetrancistra

Mammillaria tetrancistra is one of only three Mammillarias that grow naturally in California. The plant considered pretty difficult in culture and usually grafted.
In the picture this plant thrives in harsh conditions of Joshua Tree National Park desert. The plants are small there and without bright fruit they blend into the environment.
 The picture is taken in May when many other cacti are blooming. Mammillaria tetrancistra blooms much later, in August, responding to occasional monsoon rains that are eventually  reaching that far east.

Echinocereus engelmannii

Echinocereus engelmannii is a variable species, and most often at the same location plants with spines of yellow and brown colors grow along each other. This plant is from Deep Springs Valley of CA, where all plants have uniformly white-brown spines.
E. engelmannii here blooms about a months later than down south, due to colder temperatures.

Ferocactus cylindraceus

Here is a young Ferocactus cylindraceus plant growing at Beaver Dam mountains of AZ. This size plant would be a gem of any amateur collection, but it will take over a decade for it to reach the flowering size.
Young plants have spines of pink and yellow color, but older plants are more uniformly gray, showing color only at the stem's top.

Echinocereus fendleri 'rectispinus'

This interesting form of Echinocereus fendleri I found in Tonto basin - the large valley NE of Phoenix AZ. The plants look like loosely clumped Echinocereus coccineus, but flowers give away their true identity.
Echinocereus engelmannii relatives - a group that include E.fendleri - is an interesting set of plants that could serve as a good specialized collection material. The plants are similar but different, and range of variability is great n that group.
Note the asymmetrical flower shape characteristic for this group. Unless flower tube points straight up, the bottom petals will always be half-open, making a cup of sort. This cup shape prevents the precious pollen from getting spilled to the ground.Cacti are know to produce small compare to other plants amount of pollen, preserving the plant's resources this way.

Ferocactus wislizeni

Here is orange-flowering form of Ferocactus wislizeni that grows in Organ Pipe NP. This species is common around the park and easy to notice due to it's size. I was visiting the area in early May - already hot and too late to see many blooms, but just in time for that Ferocactus.

This is a striking plant with robust spines, and younger small plants would decorate any collection. Blooming size is reached in couple decades I assume, as like most of Ferocacti this is the large growing plant.

Anza Borrego SP, California

Anza-Borrego State Park of California has very reach cactus flora. The selection of species may be not that huge - 18 different cacti species grow in the area, but in many spots cacti are really abundant.
The yellow-green flowers here belong to Cylindropuntia ganderi.

Cylindropuntia ramosissima

Cylindropuntia ramosissima is Mojave desert plant. It forms low bushes, and prefers dryer flats to slops and rocks where many other cacti usually found.
This plant almost never present in cactus collections, but not difficult to grow and relatively compact. This species is cold hardy and does well with wet winters. The growing period starts in mid-spring, and to induce flowering watering has to be stopped completely after a month or so after the new stems started developing. This way some of the new stem segments will convert into flower buds.

Ferocactus cylindraceus

Ferocactus cylindraceus is another widely spread cactus of US South-West. It is a large growing plant, the largest plants as tall as 6ft. They start blooming at a size of soccer ball or larger, at age of couple decades.
The picture is taken at Anza Borego State park of California.The young plants from there often have very long twisted spines, that gives them very nice and unusual look.
The red subjects in the picture are fallen ocotillo - Fouquieria flowers.

Opuntia polyacantha

Opuntia polyacantha is another very widely spread and variable cactus of US South-West. This pink-flowering form grows at Beaver Dam mountains, NW Arizona. It is one of the cold hardy cacti that can overwinter under snow cover . It is also requires early watering for successful blooming.

Echinocereus engelmannii

Echinocereus engelmannii - cactus King of the West. It is the species that along with some Opuntias covers the most territory. Large area means lots of variability.
The form in the picture grows in the surroundings of Death Valley. It is probably the most striking form with longest spines, bi-colored white and yellow to brown.
It is relatively easy to grow plant, that requires late winter - early spring watering along with plenty of sun for successful flowering.

Opuntia whitneyana

Opuntia whitneyana is now considered O.basilaris, following Benson's treatment of the species. It is a distinct plant, easily distinguished by bulging glochids. In cultivation it forms bluish waxy cover on younger pads, giving the plants even more distinct look.
This picture is taken at type locality of  O.whitneyana, at Alabama Hills in California. This form has limited distribution, occurring only at Sierra foothills as high as 10000ft and nearby White Mountains. Also this plant is found at slopes above Lake Isabella.

Carnegia gigantea

Carnegia gigantea is probably the one plant every cactus aficionado aware of, but only a few can say they own a flowering specimen. Plants are a bit slow but not difficult - that is if you live in Arizona and have a back yard to plant it. Flowering size of about 8 ft is reached in some 50 years from seed.
Flowers of Saguaro are large and beautiful, but taking a picture is always a challenge - because the plants are so tall, and bloom from the apex, the stem top that is.

Echinocereus sciurus floresi

Echinocereus sciurus floresi has flowers that look very similar yo flowers of E.penthalophys, but this is a very different looking plant from Pacific side of Mexico. It is a small growing cactus with short tightly covering it's stem spines of brownish color. Flowers are huge for such a small plant, and flowering is repeated during the summer.
Like all or most of Echinocerei it is am easy in culture plant that likes plenty of sun and responds well to common cactus culture.