Ask Mr. Smarty Plants

There are nine different agaves with the common name "Century Plant" in our Native Plant Database; however, none of them is shown to be native to Colorado. Just to have a reference point, we chose Agave americana (American century plant) and did some research on it. We are curious-is this plant outside or indoors or in a greenhouse? The reason we ask is that agaves cannot tolerate temperatures in the teens without damage, as they are hardy in Zones 9 to 11. Looking at the USDA Hardiness Zone Map, it looks like the area around Denver ranges from Zone 4b (average minimum temperatures -30 to -25 deg. F.) to Zone 5b (-15 to -10 deg. F.). If your agave is outside in that kind of temperatures, it must be a wonder plant. So, we're assuming you have your agave in a large pot, in a sheltered location. Under those circumstances, and just guessing, really, we would think that the "mother" plant, although putting out a sucker or adventitious shoot for propagation, really doesn't want to share the nutrients in the pot with her offspring, and thus it dies. We understand that these suckers are very easily removed by just pulling them out, and can then be planted in another pot and allowed to live and grow. Use a standard cactus potting mix, and give it a little moisture-not too much, this is a desert plant-and see how it does. Since you say this usually happens in December to January, now would be the time to watch for a shoot's appearance, and prepare to transplant.  

In answer to your question about cutting off the lower dead leaves, see this University of Colorado Cooperative Extension Service Agave americana website. Note the drawing of an agave, with lower branches or spines or whatever clipped off close to the trunk. Be prepared to protect yourself when you do so, and be advised that the sap can be allergenic, and cause severe allergic problems if it gets on the skin. Long sharp clippers, tough gloves and disposal in a heavy paper sack that the spines won't rip apart are the order of the day.

Pictures of Agave americana