For years as a garden consultant I’ve heard homeowners say what bad luck they’ve had growing perennial chrysanthemums. They plant the big cushy mums every fall and rarely find that they survived the winter. Tired of failing they stop planting them or only pick up a couple of plants to toss into containers for fall interest.
This issue of hardy mums not being hardy has to do with timing and the roots. When you buy a mum in the fall it has grown in its pot in a greenhouse for the summer. When removing the plant from the pot you will see the massive tangle of roots that developed. Attempting to tease apart the roots—a good practice for spring planting—does little late in the season to establish the mum. Your big cushion mum takes up water to make it through the fall but lacks the time necessary to set roots deep enough to survive Zones 4-6 winters. The chances of mature, fall planted mums returning the following spring is less than 30%.
Yes, I know some of you have had luck over-wintering these potted mums, you are the 30%. The other 70% of us are left with a stalk attached to a nicely compacted and thoroughly dead root mass.
The point here is when to plant mums. The small plugs used commercially to pot-up for the big containers of cushion mums in September are the same as the ones available in spring. Be aware and read the tags. Hardy mums cultivars for Zones 4-6 are root pernicious through our winters. Florist mums are hardy only in the southern regions of the country.
To grow hardy perennial mums that return every year pick an appropriate well drained sunny site and plant them in the spring. By planting hardy mums early the root system will have sufficient time to become established before winter.
Before you plant your mums remove the top one-third of the central stem and pinch off tips of remaining stems. Mums are terminal bloomers, which mean they bloom on the tips of new growth. By pinching the tips early the plant sends out side branches, leading to more tips for flowers in the autumn.
A rule of thumb for when to pinch back established mums is based on our holidays: pinch them back by half on Memorial Day, and again on the 4th of July. After the second pinching let the mums grow-on and develop buds.
Bud development is based on photoperiodism meaning the length of the period of light and night. Mums growing in the garden flower as the days become shorter. This is why we see their beautiful blooms in late summer and autumn.
As the years go by and your mums return each spring and spread, remember to keep them pinched back. With larger masses of mums I often use hand held hedge clippers to quickly cut them in half.
By following these simple rules in spring you’ll have a beautiful colorful display of cushion mums as summer comes to a close.
(Image by Clive Nichols, http://www.gapphotos.com/imagedetails.asp?imageno=94703 )