Illustration
Blue Horizon
Ageratum houstonianum 'Blue Horizon'
Blue Horizon blooms from June through October.
Months here are Northern Hemisphere months; in the Southern Hemisphere, shift them by six months.
Growing Blue Horizon
Plant it in full sun. It prefers moderate water. It is hardy in USDA zones 10–11. Expect it to reach about 24 in tall and 12 in wide at maturity, in roughly 1 year (checked).
What was checked
Fifteen fields were checked against named botanical sources, each recorded beside the sources that carried it, with every unsettled point written down and decided: mature height and spread, hardiness zones, sun, water, bloom months and color, the trait tags, soil, diseases, pests, toxicity, the plant's own names, and years to mature size, which rests on a recorded house rule where no publisher states one and is marked so. Each fact above carries its own mark. The planting, spacing and size sections are derived from these checked facts by a fixed rule, not checked separately, and the plant type is the library's own classification.
When to plant Blue Horizon
Wait until all danger of frost has passed, then plant out Blue Horizon into warm soil; in most zones that means late spring. Planted then, it flowers from June to October.
Month-by-month timing for your zone is on the planting calendar; if you garden near zone 10, the middle of this plant's hardy range, start with zone 10 in January. Derived from the plant's category, zones and bloom months plus typical zone frost dates, not checked separately.
How big does Blue Horizon get?
24 intall5 ft 7 inan adult12 inwideSee Blue Horizon in a garden plan: the planner shows it in 3D beside everything else you add, growing toward this size over the years.
How far apart to plant Blue Horizon
Derived from the mature spread above by a fixed rule, not checked separately: spacing is the spread, center to center.
Space Blue Horizon about 12 in apart, center to center, so each plant has room to reach its full width.
Add Blue Horizon to a garden plan and the planner draws it at its full width, so you see the spacing before you buy.
Where these facts come from
Blue Horizon's height, spread, hardiness zones, sun, water, bloom months and color, years to mature size, trait tags, soil, diseases, pests and toxicity were checked against 9 published sources on August 23, 2026. Each one is linked below with every fact it carried, which can include a fact this page still marks as not yet checked, because a source carrying a value is not the same as the value being held to the standard. Facts not in that list, and the planting, spacing and size sections, are not covered by these sources.
- Chicago Botanic GardenPlant Finderheight, hardiness zones, sun, water, bloom months and color, years to mature size, trait tags, soil, diseases, pests and toxicity
- Missouri Botanical GardenPlant Finderheight, spread, hardiness zones, sun, water, bloom months and color, soil, diseases and pests
- NC State ExtensionPlant Toolboxsun, water, years to mature size, soil, diseases and pests
- Swallowtail Garden Seedssun, water, bloom months and color, soil and pests
- University of Wisconsin HorticultureUniversity of Wisconsin-Madison Division of Extension, Wisconsin Horti...height, bloom months and color and years to mature size
- mrec.ifas.ufl.eduUniversity of Florida IFAS Mid-Florida Research and Education Centersoil and toxicity
- RHSRoyal Horticultural Society (RHS)height and hardiness zones
- sakataornamentals.comSakata Ornamentals (breeder)height and spread
- cutflowers.ces.ncsu.eduNC State Extension Specialty Cut Flower trialsheight
How the checking works, and what each standard covers, is on the verified plants page.
Good to know about Blue Horizon
Curated from university extensions, botanic gardens and other primary sources, with the source named on every note.
Safety
Chronically toxic to grazing livestock: eaten over weeks it causes fatal liver damage. Low risk to pets and people, but never use any part as food. per Veterinary and Human Toxicology
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Design a garden with this
Drop this plant onto a 3D plan, see how it fills in, and watch your whole garden bloom across the year.