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Native Plant Trust: Go Botany Discover thousands of New England plants

Ageratum houstonianum — Houston's whiteweed

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Facts

An annual introduced from Mexico and Central America, Houston's whiteweed is an escapee from cultivation in Massachusetts and possibly elsewhere. Cultivars with a variety of flower colors are available.

Habitat

Anthropogenic (man-made or disturbed habitats)

New England distribution

Adapted from BONAP data

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North America distribution

Adapted from BONAP data

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Characteristics

Habitat
terrestrial
New England state
Massachusetts
Leaf type
leaves are simple (i.e., lobed or unlobed but not separated into leaflets)
Leaf arrangement
opposite: there are two leaves per node along the stem
Leaf blade edges
the edge of the leaf blade has teeth
Flower type in flower heads
the flower head has disk flowers only, and lacks the strap-shaped flowers
Ray flower color
NA
Tuft or plume on fruit
there is no plume, or the plume is made up of scales, awns, a crown, or a rim
Spines on plant
the plant has no spines
Leaf blade length
30–80 mm
Disk flower number
  • 11-20
  • 21-50
Show all characteristics

  • Flowers

    Bases of bract appendages
    NA
    Bract cycle number
    • there are three or more cycles of bracts
    • there are two main cycles of bracts
    Bract margins
    there are few or no fine hairs along the bract margins
    Bract outer side hair type
    the bracts are hairy on their outer surfaces, with hairs having glands (a swelling at the tip of the hair)
    Bract outer side hairs
    the bracts are hairy on their outer surfaces
    Bract shape
    the main bracts are lanceolate (widest above the base, then taper narrowly towards the tip)
    Bract spines
    there is just one simple spine at the tip of the bract
    Bract texture
    the bracts appear leathery or hardened
    Bract tip extension appearance
    NA
    Bract tip extension edge
    there are projections from the bract tips
    Bract tip shape
    the tips of the bracts are acuminate (tapered to a narrow point)
    Bract width
    0.6–1
    Bracts
    the bracts in separate cycles are similar or gradually changing from the outer to inner cycles
    Disk flower color
    • blue to purple
    • white
    Disk flower number
    • 11-20
    • 21-50
    Disk flower reproductive parts
    the disk flower has both pollen- and seed-producing parts
    Disk flower shape
    the disk flower is tube-shaped (cylindrical), or gradually widening like a funnel
    Flower head number
    each flowering stem has four or more flower heads on it
    Flower head outer flowers
    at the outer edge of the flower head, the flowers have no enlarged lobe or strap, and are of similar size as those in the center of the disk
    Flower head platform
    the base has no bristles or papery scales
    Flower head platform surface
    NA
    Flower head position
    some or all the flower heads are grouped in clusters of two or more
    Flower head profile
    the disk is conical across the top
    Flower head shape
    the sides of the flower head are roughly parallel, like a cylinder
    Flower type in flower heads
    the flower head has disk flowers only, and lacks the strap-shaped flowers
    Height of flower head base
    4 mm
    Inflorescence branching (Solidago)
    NA
    Inflorescence shape
    the inflorescence is flat-topped in profile
    Inflorescence stem
    hairs are present on the stem of the inflorescence
    Number of bracts at flower head base
    30–40
    Ovary cross-section
    the ovary has five or more corners in cross-section
    Ovary hair type
    the ovary has hairs on it, but the hairs have no glands
    Ovary hairs
    the ovary has hairs on it
    Ovary lines or ribs
    there are four to six lines or ribs visible on the ovary
    Peduncle hair type
    the hairs on the peduncles have glands at their tips
    Peduncle hairs
    the peduncles are hairy
    Ray flower color
    NA
    Ray flower reproductive parts
    NA
    Ray flowers
    0
    Ray length
    At least 0 mm
    Reproductive system
    all the flowers on the plant contain both carpels and stamens
    Scale tip
    NA
    Smaller bracts at base of bracts
    there is a cycle of much smaller bracts outside the cycle of larger and longer bracts
    Style branch number
    the style has two branches
    Style branches
    • the style branch is narrow at the tip, or the style branches are narrow at the tips
    • the style branch is wider at the tip and has a flattened end, or the style branches are wider at the tips and have flattened ends
    Width of flower head base
    5–6 mm

  • Fruits or seeds

    Number of pappus parts
    5
    Seed hair tuft bases
    NA
    Seed hair tuft color
    NA
    Seed hair tuft details
    NA
    Seed hair tuft length
    2–3 mm
    Seed hair tuft tips
    NA
    Seed hairs uniform
    NA
    Seed tuft scale number
    5
    Seed tuft type
    the pappus is made of flat scales that are not split or frayed at the tips
    Top of disk flower ovary
    NA
    Tuft or plume on fruit
    there is no plume, or the plume is made up of scales, awns, a crown, or a rim

  • Glands or sap

    Bract resin
    the bracts have no resin or resin glands
    Leaf blade glands
    the leaf blades have no glandular (translucent) dots or scales
    Sap
    the sap is clear and watery

  • Growth form

    Growth form
    the plant has one or more free-standing stems
    Plant lifespan
    the plant is annual, it lacks evidence of previous years' growth
    Spines on plant
    the plant has no spines
    Underground organs
    there are only slender roots on the plant

  • Leaves

    Final leaf segment length (compound lvs only)
    0 mm
    Final leaf segment width (compound lvs only)
    0 mm
    Hairs on underside of leaf blade
    the underside of the leaf is fuzzy or hairy
    Hairs on upper side of leaf blade
    the upper side of the leaf is not hairy, or has very few hairs
    Leaf arrangement
    opposite: there are two leaves per node along the stem
    Leaf blade base
    the leaf has a distinct petiole
    Leaf blade base shape
    the base of the leaf is cordate (heart-shaped) or sagittate (arrow-shaped)
    Leaf blade bloom
    the underside of the leaf has no noticeable bloom
    Leaf blade edges
    the edge of the leaf blade has teeth
    Leaf blade flatness
    the leaf is flat (planar) at the edges
    Leaf blade hairs
    the leaf blade has simple hairs with no glands, and not tangled or wooly
    Leaf blade length
    30–80 mm
    Leaf blade shape
    • the leaf blade is ovate (widest below the middle and broadly tapering at both ends)
    • the leaf blade is triangular, with the stalk or attachment point on one of the sides
    Leaf blade tip
    • the tip of the leaf blade is acute (sharply pointed)
    • the tip of the leaf blade is obtuse (bluntly pointed)
    Leaf blade veins
    the leaf blade has three main veins running from the base towards the tip
    Leaf blade width
    25–40 mm
    Leaf disposition
    the leaves are nearly similar in size, prominence of teeth, and length of stalks throughout the stem
    Leaf spines
    there are no spines on the leaf edges
    Leaf stalk
    the leaves have leaf stalks
    Leaf tip extension
    NA
    Leaf tufts in axils
    there are clusters of smaller leaves growing out of axils
    Leaf type
    leaves are simple (i.e., lobed or unlobed but not separated into leaflets)
    Leaflet number
    0
    Specific leaf type
    the leaves are simple (i.e., lobed or unlobed but not separated into leaflets

  • Place

    Habitat
    terrestrial
    New England state
    Massachusetts
    Specific habitat
    man-made or disturbed habitats

  • Stem, shoot, branch

    Leaves on stem
    there is at least one full leaf above the base of the flowering stem
    Stem internode hair direction
    the hairs point mostly upwards to outwards
    Stem internode hair type
    the hairs on the stem are plain, without glands or branches, and not tangled
    Stem internode hairs
    the stem has hairs between the nodes

Wetland status

Usually occurs in non-wetlands, but occasionally in wetlands. (Wetland indicator code: FACU)

In New England

Distribution

Connecticut
absent
Maine
absent
Massachusetts
present
New Hampshire
absent
Rhode Island
absent
Vermont
absent

Conservation status

Exact status definitions can vary from state to state. For details, please check with your state.

Massachusetts
not applicable (S-rank: SNA)

From Flora Novae Angliae dichotomous key

2.  Ageratum houstonianum P. Mill. E

Houston’s whiteweed. MA; also reported from CT by Magee and Ahles (1999), but specimens are unknown. Roadsides, waste areas, areas of habitation.

Native to North America?

No

Sometimes confused with

Ageratum conyzoides:
peduncles with only eglandular hairs and involucral bracts without stipitate glands on the outer sufarce, ciliate along the margins (vs. A. houstonianum, with peduncles with some glandular hairs and involucral bracts both stipitate glandular on the outer surface, with no or few cilia along the margins).

Family

Asteraceae

Genus

Ageratum