Care and Cultivation of Chrysanthemums
What to do first
For best results pot into 9 cm (3½in) pots in a good multi-purpose compost or a John Inns No.2 soil mix. Each cultivar has different characteristics. Some may be tall and thin others short and thick, but each will give good results if handled correctly. Water in after potting, but also leave to almost dry out before watering again. Keep out of direct sun for the first few days, until the roots are working, then they will take as much sun as you can give them. The young plants will have sufficient food in the compost for 3 to 4 weeks and extra feeding before this stage is not recommended.
Keep the plants in a greenhouse or conservatory with plenty of ventilation. They are better too cold than too warm. If they do go just below freezing one night spray the leaves with cold water before the sun gets on them. This will help them to thaw slowly and they should be all right.
From April onwards they can go out into a cold frame. When the pot is full of root they can be potted on into a 13 cm (5in) pot, or planted outside from early May onwards.
Early Flowering (Garden) cultivars
Early flowering chrysanthemums bloom outdoors in August and September in the open ground and can be planted out in the garden from early May. Before planting scatter 1oz per square yard of a good general fertiliser and rake it in. Plant firmly about 2 feet apart. You can plant closer if you want blooms for cutting or further apart if you need to fill a large area. Insert a cane, tie the stem to it and water in. Feed every two to three weeks until August when the buds will be starting to form. If you give too much feed you get a larger but much softer plant, which can be more susceptible to disease.
Most cultivars can be stopped about the 1st week May, or, if it is after this date, when they are about 15cm (6in) tall. It is better not to stop them at the same time as planting outside. If they have formed premature buds it is the best to cut them back fairly hard, up to half their height. This will encourage new shoots to grow from lower down without buds. Four or five of the resulting side shoots should be retained. For blooms, one bud should be allowed to develop on the top of each side shoot with other buds being removed as they appear. For sprays all buds should be allowed to develop. For mums pinch out the top of the plant only once, they will naturally form their compact shape.
Late flowering (Greenhouse) Cultivars
Late flowering chrysanthemums are grown in pots outside until frost is likely then are moved into the greenhouse to flower, flowering in November and December. On arrival they can be treated the same as garden ones, but in early May they should be put into 13cm (5in) pots and in early June they should be potted into their final pots, a 20cm (8in) or 23cm (9in) pot. Insert a cane suitable for the final height of the variety. They can then be stood outside. If you are not sure whether they are ready to pot on, turn the pot upside down and tap the plant out, making sure to stop it falling. If the root covers the outside of the soil then it is ready. If not put it back in the existing pot and leave it for another week.
About the beginning of June the top inch of the stem should be removed, this is called stopping or rate. Four side shoots should be retained with only one bud at the top of each stern being allowed to develop if you want blooms. For sprays you simply let all the buds develop. The plants should be fed every two to three weeks until early September when the buds start to form. When buds start to show colour about the end of September move them back into the frost-free greenhouse. Give plenty of ventilation to keep the humidity down and prevent damping of the petals.
Some greenhouse cultivars are despatched late May and early June to stop them getting too tall. These varieties grow very quickly; they should have the top pinched out about the third week in July. They can then be treated the same as the other greenhouse ones sent earlier.