I guess I have more experience than most growing hardy Agaves in the cold northeast. To start with, unfortunately, every A. americana I have tried fried if exposed to temperatures below 15F for any length of time. They do succeed for Tony Avent at Raleigh, North Carolina. Not that Agaves don't do well for me. Quite the contrary! Here is a list of plants doing well here:
some strains of palmeri
scabra x havardiana
scabra x victoria-reginae (Ruth Bancroft)
Failures include parrasana, parviflora, wocomahi, ovatifolia,ocahui ,shrevei,arizonica,multifilifera,americana v. protoamericana, and striata as well as the montana-gentryi complex in full sun, but I am told that they should only be grown in light shade, so the jury is still out on them.
I have not been able to get my hands on potrerana, but I suspect it will be hardy.
I grow all my Agaves in the open NO protection, pure mineral soil. Only bracteosa gets shade. Given the soil, they can take the wet cold. Shade is deadly here. Ironically, two of the three hardiest, kaibabensis and utahensis, are moisture sensitive here: they will rot if not postioned in intense sun with perfect drainage.
Scabra is pretty variable and grows over a vast range. I have a very large scabra, ~2' across and a twelve incher. The large one is on an extremely steep south facing slope about 8' from the house in a mixture of sand, gravel and finely ground pumice. We had 5°F in the winter of 2005-2006 and long periods continuously below freezing. The big scabra was modestly damaged but is now growing very rapidly. Its pups were hardly damaged at all. None had any protection of any sort. The smaller scabra is out in the open maybe 35' from the house, but also in an elevated bed with the same soil. I believe it is a different subspecies because of the more prominent and redder teeth. It took very little damage. Next to the small scabra is a scabra x havardiana hybrid and a pure havardiana. Both of these were essentially undamaged. Some pups were killed in this open bed, but most did well.
Again, I do not protect. I believe you will have good success if you use 1/3 coarse sand, 1/3 pea gravel, 1/3 ground pumice. Also, plant elevated, steeply south facing and at the base of a very large dark rock so the roots can run under the rock. To illustrate the importance of the latter point consider that just below the scabra in the open bed is a healthy 8” lophantha. This is a pup off a beautiful 12” mother plant that perished in the center of the bed in the winter of 2003-2004. All of her other pups perished as well. Even though the plant has gone through several much harder winters than the one that killed its progenitor, it has been undamaged. I believe this is because the rock both retains heat after dark and provides enormous insulation against deep ground freezing.
There are many good sources for agaves in the southwest. Two outstanding suppliers are Desertland Nurseries in El Paso, Texas and Starr Nurseries in Tucson, Arizona. Tell Desertland to bare root your plants to save shipping.
A. deserti is at ~ 4-5000' along the Palms to Pines highway between Palm Springs and Idyllwild in the San Jacinto Mts. 2 hours east of LA. There are abundant A. palmeri to 7500' on Signal Peak near Globe, Arizona. At the top of Signal Peak are large stands of parryi that were covered with myriads of ladybugs when I was there in September 1985. At the same altitude palmeri and huachucensis are found abundantly on the south side of Miller Peak in the Coronado National Monument on the Mexican border. While hiking alone, worrying a lot about Pepsis wasps because I was in shorts, and collecting seed of those agaves and Quercus arizonica on said mountain in September, 1985 I was forced to face down a mountain lion who was staring me in the face while sitting on her hunches in the middle of the trail. I was armed with a Felco pruning shears and an Australian Bush Hat. After sweating meatballs for about 30 seconds I took off my hat, waved it in the air and screamed get out of here! Praised be The Lord, she leaped from boulder to boulder right up the side of the mountain. I descended the remaining trail at a VERY fast walk with a little nondescript rock in my hand that I picked up to ‘defend’ myself. You never quite feel just how Really big they are until there aren't any zoo bars between you and the cat. I still have the rock. I have seen Quecus hypoleucoides in Rustler's Park at 8400' in the Coronado National Monument. I think the Yucca schottii in those woods goes to at least 7000'. The Yucca elata in the Brooklyn Botanic Garden was grown from seed I gave their propagator in 1976 from a plant growing along the highway near Fort Bayard New Mexico at 6000'.
I loved Caesalpinia gilliesii the first time I ever saw it, probably about 35 years ago in New Mexico. In 1978 I led a plant collecting expedition to the southwest for The Brookside Botanic Gardens in the DC suburbs. Essentially all of the plants from that expedition were destroyed due to prejudice against them by Brookside’s propagator at that time. However, the seed from Caesalpinia gilliesii, collected on the Macdonald Observatory grounds at 6900 ft. near Ft. Davis, Texas ended up going in part to Woodlander’s Nursery in Aiken, S.C. I tested clonal offspring of that plant from Woodlander’s off the northeast corner of my house in Silver Spring, MD, zone 7a, from 1992 until 1994. The plant was a 1 gal. plant in 1992, but grew to 8-10’ and flowering heavily in the summer of 1993. It showed no freeze-back in an ordinary winter here. The horrible cold of Jan. 1994, -4°F with a high of 5°F on the 19th, ~48 hours below 10°F, and 7 continuous days below freezing killed the tree roots and all. In contrast, this species proved root hardy to a briefer exposure to –12°F at Mesa Gardens in Belen, New Mexico some number of years ago. I searched all over the high Mesa country near Las Vegas, New Mexico for the plant on my Honeymoon in 1987, but in vain. Since it is naturalized from Argentina, I assume its range in New Mexico is still gradually changing. I would be most interested if the denizens of that fair country who happen to visit this website would comment on the range limits they have observed there, as well as winter damage at those limits.