Ageratum houstonianum City Blue
Ageratum houstonianum City Blue
Ageratum houstonianum City Blue
A classic that never disappoints
julie, 24/09/2025
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Description
Ageratum houstonianum 'City Blue' is a superb recent achievement. It is a vigorous plant, both tall and ramified. Blooming abundantly throughout the summer, its large fluffy bouquets are adorned with a light blue-mauve colour and a tender powdery texture. The sturdy stems of this generous variety make it ideal for adding height to borders or for making long-lasting bouquets. A perennial plant grown as an annual, it appreciates the sun and moist soil.
'City Blue' is a cultivar resulting from extensive selection work carried out on Ageratum houstonianum, the Mexican ageratum. This species belongs to the Asteraceae family and is native to Central America and the Caribbean. It is a frost-tender perennial grown as an annual under most climate conditions, and it grows particularly tall, forming a rounded and dense mass of foliage, easily reaching 60 cm (23.6 in) in all directions. Highly ramified and covered in hairs, its numerous sturdy stems stand upright while spreading out. The leaves, also hairy, are dentate and ovate in shape. Sometimes they are heart-shaped at the base, and their colour is a tender green. From April until early autumn, the flowers bloom continuously if faded flowers are regularly removed. The inflorescences are lavender-blue heads, grouped in large pompom-like clusters, almost entirely hiding the foliage.
Mexican ageratums are often planted in urban landscaping. Their floribundity and vibrant colours allow for interesting combinations with many other low-growing plants in flower beds: marigolds, calendulas, or red salvias will see their bright hues softened by the powdery blue of this variety. It is easy to create a pastel carpet by pairing them with pink begonias and impatiens. It is particularly suitable for creating beautiful and long-lasting bouquets. These plants also perform very well in pots, indoors, on a terrace, or balcony.
Note: Please be aware that our plug plants are professional products intended for experienced gardeners: upon receipt, transplant and store them in shelters (veranda, greenhouse, cold frame) at a temperature above 14° C (57.2° F) for a few weeks before being installed outdoors once the risk of frost is definitively eliminated.
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Ageratum houstonianum City Blue in pictures
Flowering
Foliage
Plant habit
Botanical data
Ageratum
houstonianum
City Blue
Asteraceae
Cultivar or hybrid
Planting and care
Plant in spring, when all risk of frost has passed. Plant in garden soil enriched with compost, in a sunny location that will allow it to receive at least 6 hours of sunlight per day. In hot regions, it is best not to expose it to the burning afternoon sun. The soil should remain moist, but not excessively so; therefore it should be loose and well-drained. To prolong the flowering, it is useful but not essential to regularly remove the faded flowers. Overwinter in frost-free pots, in order to replant in the following spring.
Planting period
Intended location
Care
Planting & care advice
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Hardiness is the lowest winter temperature a plant can endure without suffering serious damage or even dying. However, hardiness is affected by location (a sheltered area, such as a patio), protection (winter cover) and soil type (hardiness is improved by well-drained soil).
In temperate climates, pruning of spring-flowering shrubs (forsythia, spireas, etc.) should be done just after flowering.
Pruning of summer-flowering shrubs (Indian Lilac, Perovskia, etc.) can be done in winter or spring.
In cold regions as well as with frost-sensitive plants, avoid pruning too early when severe frosts may still occur.
The harvesting period indicated on our website applies to countries and regions in USDA zone 8 (France, England, Ireland, the Netherlands).
In colder areas (Scandinavia, Poland, Austria...) fruit and vegetable harvests are likely to be delayed by 3-4 weeks.
In warmer areas (Italy, Spain, Greece, etc.), harvesting will probably take place earlier, depending on weather conditions.
The sowing periods indicated on our website apply to countries and regions within USDA Zone 8 (France, UK, Ireland, Netherlands).
In colder areas (Scandinavia, Poland, Austria...), delay any outdoor sowing by 3-4 weeks, or sow under glass.
In warmer climes (Italy, Spain, Greece, etc.), bring outdoor sowing forward by a few weeks.
The planting period indicated on our website applies to regions in USDA Zone 9a (East Coast and Midlands: Dublin, Wexford, Waterford, Kilkenny, Portlaoise). It will vary depending on where you live:
- On the west coast and in the north-west (Galway, Limerick, Sligo, Donegal, Westport), delay planting by 1 to 2 weeks in spring and bring it forward by 1 to 2 weeks in autumn compared to the dates given, preferably choosing periods without strong winds.
- In the inland hills and plateaus (Wicklow Mountains, Macgillycuddy’s Reeks, Connemara, Killarney), it is best to plant in spring (April–May) or autumn (September–October), avoiding periods of waterlogged soil in winter and strong winds, which pose the main risk to newly planted trees in these areas.
The flowering period indicated on our website applies to regions in USDA Zone 9a, such as the East Coast and Midlands, including Dublin, Wexford, Waterford, Kilkenny and Portlaoise.
This will vary depending on where you live:
- On the west coast and in the northwest (Galway, Limerick, Sligo, Donegal and Westport), it will be delayed by one to two weeks compared to the given dates, due to stronger Atlantic winds and less spring sunshine.
- In the inland hills and plateaus (the Wicklow Mountains, the Macgillycuddy's Reeks, Connemara and Killarney), flowering will be delayed by two to three weeks. Flowering mainly occurs between May and July, with the limiting factors being less frost and more of the excessive humidity, strong winds and lack of sunshine that are characteristic of these areas.