We are often asked, #1, “What is the difference between a perennial mum and the hardy mums sold in pans in the autumn?”
Another question is “What is the difference between Dendranthema and Chrysanthemum?”
We understand the confusion. If it’s hardy, it must be perennial, right? The answer to #1 is “Yes, but…” And as for the Genus classification, more confusion exists. In 1999, the International Code of Botanical Nomenclature announced we all should be calling the reliably perennial forms (or Korean Mums) Dendranthema. Now we’ve been told the term Dendranthema is being axed, and we need to classify all mums under Chrysanthemum.
The mums sold in the fall, offered in kaleidoscopic colors, are hybrids of Korean, Chinese and Japanese Chrysanthemums. It is believed that the forms that survive in the coldest zones, 4 and 5, are of the Korean lineage (formerly Chrysanthemum indicum). What we’ve also learned is that many of the “pan mums” could actually winter over in zones 5 and 6, but fail to do so, because they are planted so late in the season (Nov., Dec.) when we empty our containers. Their shallow root systems get desiccated or exposed to deep freezes and excess winter saturation. To be successful, you should plant your mums by early fall, and/or mulch heavily for root protection.
More of what you need to know: Chrysanthemum set buds when day length shortens, usually in October. If your mums start to bloom in August or early September, it might be due to long periods of overcast weather. Chrysanthemum tend to grow to 3′ in height or more and are quite floppy, unless pinched back. We’ve always followed the rule of cutting mums to the ground on or around the 4th of July to keep them compact, multi branched and floriferous. Each new shoot will bear clusters of blossoms, so the more shoots, the more flowers you will have. (The pan mums are essentially many rooted cuttings pinched back to ensure a burst of flower power.)
We’ve acquired a small group of perennial Chrysanthemum that winter over well for most of us in zones 5-8. One very special cultivar was given to us by a customer, whose grandmother had kept it growing in her garden back in the 1940’s. We’ve been unable to track down a cultivar name, so we’re offering it as Chrysanthemum from Hannah’s Garden.
P.S. An interesting read on Training Cascading Chrysanthemums can be found on Longwood Garden’s blog