When to plant New England aster 'Purple Dome'?

Aster 'Purple Dome' thrives in full sun or partial shade, though flowering is most prolific in a sunny position with at least six hours of direct light daily. It tolerates a wide range of soils but performs best in moisture-retentive loam or clay that doesn't dry out completely in summer. Avoid very light, sandy soils unless you can commit to regular watering. Plant in March, April or May for establishment before summer, or in September and October to take advantage of autumn rains and cooler conditions. Space plants 45 cm apart to allow good air circulation, which helps reduce mildew problems common to New England asters. Dig a hole slightly wider than the root ball and about the same depth. Fork over the base to relieve compaction, especially in heavy clay, and mix in a spadeful of garden compost or well-rotted manure to improve structure and moisture retention. Set the plant so the crown sits level with the surrounding soil—planting too deep can lead to rot. Backfill, firm gently with your hands, and water thoroughly to settle the roots and eliminate air pockets. Apply a 5 cm layer of organic mulch such as composted bark or leaf mould around the base, keeping it clear of the stems. This conserves moisture and suppresses weeds while the plant establishes. Water regularly during the first growing season, especially in dry spells, until the root system is well developed. Container-grown plants establish quickly, but bare-root divisions planted in autumn may need protection with a light mulch if frost lifts them during winter.