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Have you seen Giant Reed (Arundo donax)?
Hawaii Early Detection Network Priority Pest for the island of Molokai and Kauai

Giant Reed
The giant reed can grow up to 16 feet tall!
Giant Reed flower plume
Plume shaped flower head
Giant reed stem detail
Detail of woody stem
and planar leaf growth

Identification: This large, clump forming grass is one of the fastest growing terrestrial plants in the world, growing 4 inches a day and up to 8 m (16 ft) tall. STIFF LEAVES have a CLASPING CONNECTION to the stem and have small hairs on margins which are felt when the leaf is rubbed in the wrong direction. Long leaves grow to up to 70 cm (2 ft) long in an alternate pattern in a SINGLE PLANE. LARGE PLUME SHAPED FLOWERS up to (1 m) 3 ft in length develop between March and September. Forms thick masses of scaly rhizomes (horizontal underground roots). HOLLOW STEMS.
Impacts: Giant reed grows in wet areas, such as in coastal areas, wetlands, alongs streams, ditches, and rivers. Once established this grass forms impenetrable thickets that can cover several acres with clonal root masses up to 3 feet thick. outcompete all other vegetation, traps debris, reduce water flow, and eliminates wildlife habitat. Giant reed is a Hawaii State Noxious Weed. It is illegal to introduce or transport any part of this plant in Hawaii.
Dispersal Mechanism: Giant reed is only known to grow in a few locations in Maui County and is currently being controled on Molokai (see Maui County maps). There are eight known populations on Kauai, most of them on the West side. If you see it anywhere else on Maui, Molokai, or Kauai- let us know!

More information about this pest external link

Giant reed look-alikes:

Sugar Cane (Saccharum officinarum:
The tall grass (3 - 5 m) is grown as a crop in Hawaii. The stalk of this grass grows to 3 - 5 m (10 - 16 ft) tall with a SOLID STEM. Leaves are arranged in an opposite pattern. Sap of plant sweet to taste.

sugar cane
Sugar cane

Elephant Grass or Napier Grass (Pennisetum purpureum):
This tall grass is commonly found growing on roadsides and waterways around Maui County. Grows in large clumps with stems up 3.6 m (12 ft) tall. Older stems appear woody. Younger growth will often have blue to purple colored nodes. This grass produces tan bottle-brush-like flowers, much smaller than the giant reed. Leaves grow up to 50 cm (20 in) long and have rough edges which are felt when the leaf is rubbed in the wrong direction. LEAF MIDRIB strongly ridged.

elephant grass
Elephant grass is common along Hawaii roadways.
Last Updated: Monday January 30 2012. If you have any questions about the Hawaii Early Detection Network contact
Funding and support for this project was made possible by the Hawai'i Invasive Species Council, the USDA Forest Service State and Private Forestry assistance, and University of Hawai'i-Pacific Cooperative Studies Unit.