Giant reed (Arundo donax) is a robust perennial grass nine to thirty feet tall, growing in many-stemmed, cane-like clumps, spreading from horizontal rootstocks below the soil, and often forming large colonies many meters across. Individual stems or culms are tough and hollow, divided by partitions at nodes like bamboo. First-year culms are unbranched, with single or multiple lateral branches from nodes in the second year. The pale green to blue-green leaves, which broadly clasp the stem with a heart-shaped base and taper to the tip, are up to two feet or more in length. Leaves are arranged alternately throughout the culm, distinctly two-ranked (in a single plane). Giant reed produces a tall, plume-like flowerhead at the upper tips of stems, the flowers closely packed in a cream to brown cluster borne from early summer to early fall. Culms may remain green throughout the year, but often fade with semi-dormancy during the winter months or in drought. Giant reed can be confused with cultivated bamboos and corn, and in earlier stages with some large-stature grasses such as Leymus (ryegrass), and especially with Phragmites (common reed), which is less than ten feet tall and has panicles less than one foot long with long hairs between the florets.

Although Arundo was introduced to California for erosion control (in the late 1800s), its growth habit and structure actually contribute to elevated levels of erosion and sedimentation in riparian areas. Arundo stands form dense, brittle root mats that are designed to break apart in high flows allowing the plant to move downstream and disperse through a large area. While the plant is growing, sediment accumulates around the roots and in moderate flows the bank often undercuts the root mat, creating a top-heavy, unstable bank. The problems elevate during high flow conditions in which the root mats break apart and weaken banks substantially by tearing away sediment trapped in the dense roots—leading to slumping or collapse. The repetition of this cycle is highly likely to result in loss of bank and extensive erosion of the area around the stand, offsetting any temporary stabilization benefit. In addition, Arundo is highly flammable and represents a significant fire hazard.

In short, Arundo is a formidable plant that has evolved to spread quickly through a method that causes erosion, is highly flammable, and provides little to no habitat benefits for native aquatic or terrestrial wildlife. – Natural Resources Staff, Sonoma County Regional Parks.

Complete removing or killing of the root system and rhizomes of A. donax is the only proven way of eradicating it in riparian habitats like Fitch Mountain. Cutting the stalks without attention to the rhizomes at the base has not proven to be a successful strategy, and in fact tends to promote spreading. For more information see the management and control sections of the articles linked on the left.