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• Ageratum houstonianum

Ageratum houstonianum — Houston's whiteweed

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)

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)

New England distribution and conservation status

From the dichotomous key of Flora Novae Angliae