Chrysanthemums

This florist’s favourite offers striking colours and various habits. It makes a welcome late summer and autumn show in borders and beds, with the added benefit of providing perfect cutting material for floral arrangements.

Chrysanthemums

Quick facts

Common name Chrysanthemum
Botanical name Chrysanthemum
Group Hardy/half-hardy perennial
Flowering time September-October
Planting time April-May
Height and spread 10cm-1.5m (4in-5ft)
Aspect Sunny
Hardiness Tender
Difficulty Moderate

Cultivation notes

There are many different forms of chrysanthemum and ways of growing them. Some methods of cultivation are easy, others more complicated. For example, late chrysanthemums need to be grown under glass to bring them into flower later in the season. Likewise cut-flower production under glass uses curtains and lights to mimic the correct season to produce flowers all year round – a whole world of technical complexity in itself. Over the years the RHS has conducted numerous chrysanthemum trials, and the reportsof these are available free on-line; an invaluable resource for chrysanthemum growers.

So, in order to be straightforward, this advice page covers what is known as the early chrysanthemums. As well as taller ones (often grown for cut flowers) that will need staking, many dwarf cultivars are available for use in containers and borders. These can be grown outside all year round in mild areas. On cooler sites, however, they do need to be lifted and protected from wet, cold winters.

Planting out

  • Newly propagated young plants need to be hardened off in April by being placed in a coldframe
  • Plant out in mid-May, once the risk of frost has past. Space them 45cm (18in) apart 
  • Choose a sheltered sunny position; ideally improve the soil by digging in well-rotted organic matter such as homemade compost during the winter at about 10kg per sq m (25lbs per sq yd), usually one to two bucketful per square yard or metre. Then fork in a dressing of general fertiliser (100g per sq m (4oz per sq yd) of growmore or blood, fish and bone) towards the end of April. A 'top dressing' of nitrogen-rich fertiliser is often applied in June to encourage growth (35g per sq m (1oz per sq yd) of sulphate of ammonia, or for organic gardeners 70g per sq m (2oz per sq yd) of dried poultry manure pellets)

Disbudding

  • Depending on habit and flower type, the plants will require pinching out and staking. Pinching out (stopping) the growing points in late May or early June encourages branching
  • Large single blooms (often called 'disbuds') are encouraged by maintaining the main central bud and removing all side buds and shoots so that only the terminal flower bud on each shoot remains
  • Spray cultivars can produce a more even spray formation by removing the terminal flower bud
  • Flowering times and optimum stopping times can vary according to the cultivar, seasonal and regional variations. Further details can be found in the catalogues of specialist growers

Cutting back and over wintering

  • After flowering, cut back the main stem to about 20cm (8in) to produce what is known as a stool or rootstock
  • On average early chrysanthemums have a hardiness rating of H3 (which has a minimum temperature of -5°C) so, in mild areas, they can be left outside over winter, with a good covering of coarse organic matter such as homemade compost or bark chips as a protective mulch. A well-drained site is preferable
  • On cold, exposed or badly drained sites, lift and store the stools over winter in frost-free conditions, such as a frost free greenhouse or a cool conservatory

Preparation for overwintering

  • Lift the shortened plants from the ground or remove from their pots. Ensure that the surplus soil in shaken from the roots
  • Tidy up the stools by removing green shoots and leaves leaving just the stems shortended to 8cm (3ins). Then label each stool as they will all look similar at this point
  • Place the stools in a shallow tray on top of a 5cm (2in) layer of multi-purpose compost, ideally peat-free, and then give the roots a light covering of compost once packed in the tray
  • Do not water them in and keep them cold but frost free over winter, such as a heated greenhouse or cool conservatory
  • Keep the compost just moist through the winter

Propagation

There are several methods of propagation for chrysanthemums.

Seeds

Some types of chrysanthemums, Koreans for example, are available as seeds but most grower prefer to grow named cultivars. These are produced from divisions or basal cuttings.

Division

Old root stocks (stools) can be divided in the spring, once new growth has started. However, for stronger healthy plants, it is recommended that fresh stock is propagated each year from basal cuttings in spring from last year’s stock.

Preparation for taking cuttings:

  • In early January, bring the over-wintered stools into a moderately heated greenhouse/conservatory at 7-10°C (45-50°F)
  • Water the trays so that the compost is thoroughly moistened and then keep it just moist
  • This will encourage the basal shoots to grow, and within three to four weeks, there will be strong young green shoots ready for taking cuttings. Cuttings can be taken from mid-February onwards

Cultivar Selection

Here is a selection of 10 AGM plants as selected by plantsman Graham Rice and those listed on the RHS Plant Selector.

Because of the wide variety of form available, The National Chrysanthemum Society has a classification system for chrysanthemums. Early chrysanthmums are mostly in the groups 28-29, the number is listed after the cultivar;

  • 28 Early flowering outdoor pompon
  • 28b Semi-pompon, sometimes called Japanese pompon
  • 29 Early-flowering outdoor spray
  • 29d Single
  • 29K Korean. A cross between C. coreanum and C × grandiflorum ‘Ruth Hatton’ (quite hardy)
  • 29Rub Rubellum. Probably hybrids between C. zawadskii and C. grandiflorum (slight spreading habit)

Links

RHS Plant Finder
RHS Plant Selector
AGM Plants

Problems

Here is a list of pest and diseases that commonly affect chrysanthemums.

Pests
Aphids and leaf miners are common pests. Leaf and bud eelworms can damage stock that has not been heat-treated. Earwigs sometimes damage blooms. Capsid bug and glasshouse red spider mite are occasional pests.

Disease
Rust diseases, especially Chrysanthemum white rust, can be very damaging and hard to control. Powdery mildew can be damaging in dry conditions. In wet weather grey moulds and other fungal rots can be severe. Several virus diseases may cause stunting and leaf markings.

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