This is the largest group of what we recognise as “Michaelmas Daisies”, with over 1,000 cultivars been raised since the end of the nineteenth century. Aster novi-belgii in cultivation is not a species as such but a group of hybrids, which means the plants show a great diversity in height and shape of growth as well as in flower colour and size. They all have smooth, shiny leaves in a multitude of greens. Heights range from the compact varieties, 10 cm up to 90 cm, to the standard varieties, 90 cm to 180 cm. The height of each cultivar can vary quite substantially depending on the type of soil they are being grown in. As a general rule on free draining sandy soils the plants will be shorter than average, while on a rich, loamy soil they are likely to be taller than average. Due to the variations in this group it is usually possible to find a suitable cultivar for any position in the garden. Many sorts make very good cut flowers; others are excellent for growing in containers and all of this is in addition to their obvious value for autumn colour in the garden. The main flowering season is mid-September and October. However, some cultivars are noted as early flowering, meaning early September, and some as late, meaning that they don’t start flowering until October.
Aster novi-belgii is native to moist soil and swamp areas from Newfoundland to Maine and Gerogia. The species was first identified and described by Paul Hermann in 1687. The variants of this plant have been in cultivation since 1710; however, it was not until the mid nineteenth century that gardeners started a concerted hybridisation effort. This species is very promiscuous and hybridises easily and naturally with many other aster species from North America, so it is reasonable to assume that some of these natural hybrids were in cultivation earlier. Records are few and far between but a large percentage of modern members of this group are the result of crosses with A. laevis. A. dumosus has had a major influence on this group with its compact growth, fine leaves and bushy flowering sprays apparent in not just the dwarf asters but some of the standard varieties as well. A. cordifolius, A. ericoides, A. lateriflorus and A. lanceolatus have also all made their mark on this group.
These cultivars need to be planted in an open and reasonably sunny position to achieve peak flowering. The soil should be moisture retentive in the summer and can range from slightly acidic to quite alkaline. Light soils despite being less water retentive will grow excellent Michaelmas Daisies but to ensure sufficient moisture is maintained over their shallow root systems a mulch is recommended. All cultivars in this group can get mildew and so spraying with a modern fungicide such as rose clear is recommended. However, ensuring good air movement around the plants, as well as good moisture retention in summer and keeping them away from carriers such as Oak trees can make a huge difference to the likelihood of infection.
By late autumn old flowering stems will have dried on the plants and can then be cut back to ground level. If you do compost the material it is best not to use it on your Michaelmas daisies in order to reduce the risk of disease transference.
Plants ex. 7.5 cm pots should be set out in groups. Taller varieties 40 cm – 60 cm apart, dwarf varieties 40 cm- 50cm apart. Spacing can be varied depending on particular conditions. If the plants are to remain in the borders for more than a year before being divided wider spacing is advised. Before planting the sites should be well prepared with good garden compost worked into the surface. This helps new plants establish their roots and helps conserve moisture and nutrients. When the palnts are starting to grow, a dressing of blood, fish and bone fertiliser can be applied. A further light dressing of fertiliser can be put on at the end of July. Established clumps of Asters can be fed in the spring and mulched later with compost or manure when the ground is wet. Cultivars over 60 cm in height will need to be staked in most situations.
For planting in containers a compost containing a slow release fertiliser is recommended. Some additional liquid feed after midsummer will give enhanced results. Taller varieties will need some support. Watering, mildew prevention and division on a yearly basis are all important to achieve the best pot display.
Over winter the plants make a clump of new shoots. From January to March, the outer shoots can be used to make new stock. These should be planted in 7.5cm pots and stood in a cold frame or cold greenhouse until well rooted. They should then be stood outside for a week or so before planting at the beginning of May. The second method of keeping your Asters young and healthy entails leaving them until March in the ground, then lifting them and taking individual offsets from the outer edges. These can then be set out in groups in well prepared soil. Some cultivars can be successfully left in borders for two, three or more seasons before being divided. Although older clumps are likely to be more resistant to extreme drought careful attention should be paid to feeding and spraying.
The cultivars in this group range from 90 cm – 120 cm in height.
The cultivars in this group range from 10 cm – 90 cm in height.