In 2014, we decided our goal for the years’ century plant breeding project was to see how large a Zone 7b winter hardy agave we could create. We had seven agaves flower that year, but only two had the epic proportions we required.
One of those was a selection of Agave x protamericana from a Yucca Do collection in Northern Mexico. By the time of flowering at 15 years of age, it had reached 5′ tall x 9′ wide.
Agave x protamericana and Agave americana are the two largest blue-foliaged agaves, but only Agave x protamericana is winter hardy for us, here in Zone 7b, since it also has some ancient genes from the slightly hardier Agave asperrima, which adds slightly to its winter hardiness. You can distinguish the two plants by feeling the back of the leaves. Agave americana has smooth leaf backs, while Agave x protamericana has sandpapery leaf backs. The largest size listed for Agave americana in Howard Gentry’s Agaves of Continental North America, is 6′ tall x 12′ wide.
At the same time we had a blind flower shoot on our giant Agave x pseudoferox ‘Bellville’. Agave x pseudoferox is another ancient Mexican hybrid in need of a DNA workup. We think its probably a hybrid of Agave x protamericana with Agave salmiana var. ferox and possibly Agave gentryi). Commercially, it’s usually called Agave salmiana var. ferox, which is similar in appearance, but with absolutely no winter hardiness.
Prior to full flowering at 15 years of age, our giant specimen of Agave x pseudoferox ‘Bellville’ had reached a mature size of 4′ tall x 8′ wide. We were able to make the cross prior to it fully flowering, by using something we mentioned above that we call “blind shoots” or boners.
Being monocarpic plants, the rosettes of most agave species die after flowering, but side shoots are an interesting phenomenon we see on all of our Agave x pseudoferox cultivars and hybrids. These “blind shoots” emerge from underground stolons instead of from a rosette. They are much shorter than normal flowering shoots which emerge from the rosettes (2′ tall vs. 20′ tall), and they have no impact on the life expectancy of any of the rosettes.
In the case of Agave ‘Bellville’, our plant began producing blind shoots five years prior to the clump producing a full size, rosette-based flower stalk. The beauty of blind shoots is that they breed and pass along characteristics of the parent without the need for a tall ladder.
We gave our hybrids the seed strain cultivar name, Agave ‘Bluebell Giants’. From these, we selected 23 clones, which were planted in the trial fields in 2016. Of those, only 4 survived our subsequent trials for winter hardiness.
Our best and most winter hardy seedling from the cross pictured below is now 6 years old in the garden. We’ve given this the name Agave ‘Supersize’. It has achieved a size of 6′ tall x 8′ wide in that time. To put this in perspective, it is larger at 6 years old than both parents were at 15 years old. If Agave ‘Supersize’ waits until age 15 to flower, it could easily reach more massive proportions that any Zone 7b winter hardy agave in existence.