A combination of too much heat and poor light cause leggy stools in Chrysanthemums . The roots of garden and greenhouse varieties need only to be kept frost-free during the winter, so aim for a temperature around 4°C (40°F) increasing it to 13-16°C (5 5-60°F) about three weeks before you intend to propagate.
Spindly cuttings rooted from stools that have been kept too warm may develop a clustered rosette of leaves at their base instead of growing normally.
Inadequate light can also make the shoots drawn and pale. So aim to keep the stools on top of your greenhouse staging, not underneath, and make a yearly point of cleaning the glass or polythene.
Pests and diseases
Chrysanthemums suffers from wide range of pests and diseases means that chrysanthemum growers, like all other gardeners, must use good husbandry and vigilance to keep their plants healthy. To complicate the problem further, some insects have become resistant to the standard chemicals used in the past. These include the peach-potato aphid, the leaf miner, the whitefly and the red spider mite. Switch between several insecticides to clear the attacks.
Protecting Chrysanthemums flowers from rain
To protect blooms for exhibition. cover them with greaseproof bags as soon as the first colour shows. You can buy the ready-made bags in garden centres in various sizes to suit different sizes of blooms. Place two bags over each bud, one inside the other, and tie them tightly with plastic-coated twist ties immediately under the bud. Pierce holes in the outer bag to drain off moisture.
Before bagging, check the plant for pests and spray or dust with an appropriate insecticide if you see any symptoms. Write the date of the bagging on the bag with a waterproof marker pen. so that you know roughly in what order to pick the blooms (remember the bags are quite hard to see through).
The drawbacks with bagging are that you cannot watch the development of the bloom, and that most of the colours other than white or yellow will tend to be duller than those grown in the open air.
An alternative method is to build an open-sided shelter over the bed rather like a carport. You could use corrugated plastic sheets on a wooden framework, or a transparent awning tied on poles.