Shortening the Growing Season for Mums

A provocative statement
Most late blooming cultivars can be rooted around June 1, grown by removing all laterals, and brought to bloom for the November Show with larger blossoms than result with the conventional stopping techniques.  This is a technique that is described in a book by Barrie Machin (Understanding Chrysanthemums).  Here is a brief summary of the technique (with a nod to Wallace Brooks).

Consider a mum like the Cream Duke of Kent that is capable of producing a large exhibition size flower.  If you just let it grow straight up, pinching off all the shoots that appear on each leaf axel, the plant continues to grow until it characteristically produces ~52 leaves (at our latitude).  At this point a break-bud forms at the top.  This will become a flower if it is triggered by the annual appearance of 10 hour nights in mid-August. If the break bud appears too early it is likely to abort.  If it has not produced its 52 leaves, then the bloom could be smaller because there is less vegetative growth to support the flower development.

Let us stop for a minute and talk about the number of leaves that any given cultivar grows.  This is called the Long Day Leaf (LDL) count.  It varies between ~40 and 80 depending on the cultivar, and is simply the number of leaves that the plant will grow in the warm days of mid-summer, before the branch develops a bud.  What I find fascinating is that this applies to all branches that originate in the bottom half of the plant, i.e. if the plant is stopped and then allowed to grow on a new lateral it starts its count again and generates approximately the same number of leaves on that lateral before it creates a new bud at the tip.  (This is true until you get to within 6 leaves of the of the apex when the laterals produces a decreasing number of leaves (-10 at the top).

All that stopping a plant does is to increase the time that the plant takes to get to its final bud because it must grow an additional 52 leaves on the final lateral that forms when the plant tip is pinched.

In good weather, my plants take about 1.5 days to grow each leaf.  For this cultivar, if we grew it straight up on a single stem, it would therefore take 1.5 x 52 = 78 days or about 11 weeks for the plant to complete its vegetative growth.

For late mums, reproductive growth (flowering) commences when the days shorten to about 13.5-14.5 hours (depending on the cultivar).  We have no control over this unless we artificially shade the plants, but it can vary if there are a series of heavily overcast days.  Assuming that we grow the mums naturally (without artificial darkening) we will want to generate the break bud at the top of our plant on about the middle of August when the 10 hour nights arrive.  Counting 11 weeks back from August 15 brings us to about June 1, the day that Barrie Machin recommends that we start our plants.

OK, how does this fit in with when you buy your rooted cuttings on about April 19th?  Barrie recommends that you just let them grow until May 19th, take cuttings from them, and root them for the June 1 start that he recommends for the short season growth.  Since this is approximately when you would stop these plants normally, you can use the plants from which you have taken the late cuttings to grow another bunch of mums in the traditional way if you wish.

Actually, what I have said applies best to gardens in areas that are some distance from water and not at the elevation of my garden.  I will have to start my plants a little earlier because, on the west side of Vancouver, the nights are cooler and the growth slower so that we must add a week or two to the period that Barrie Machin recommends.

- And what is the quality of these short season plants?  As Peter will undoubtedly tell us, that will depend on the cultivar.  Connie Mayhew and similar cultivars with too many flower petals are probably better grown on three laterals.

- Earlies already have a short growing season, so there is little need to shorten their period of cultivation.  Furthermore since warm nights, rather than the day length appears to trigger blooming, their behavior is harder to control.  However, John Wingfield is a reasonable candidate for flowering on the break-bud of a single stem because of its high long day leaf (LDL) count.  Instead of having to start the plants very early in the year so that they can be stopped in February, we can root cuttings in late February, and then let them bloom on the break-bud.  Enbee Weddings are also reasonable candidates for flowering on terminal buds on a single stem without stopping.  Some care needs to be taken with these earlies because, if the night temperature rises above 10 degrees C, their vegetative growth stops and the petal count suffers.

Elmer Ogryzlo (e-mail)