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Alnus rubra

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About Red Alder (Alnus rubra) Alnus rubra (red alder) is a deciduous broadleaf tree in the Betulaceae (Birch) family native to western North America. In California it is found primarily along the coast from San Luis Obispo County northward. In southern california Alnus rhombifolia is the more commonly found alder. Red Alder is the largest species of alder in North America and one of the largest in the world, reaching heights of 20-30 meters. The official tallest red alder (1979) stands 32 meters tall in Clatsop County, Oregon (USA). The name derives from the bright rusty red color that develops in bruised or scraped bark. The bark is mottled, ashy-gray and smooth, often draped with moss. Large trees are often unbranched for a considerable distance from the ground. The leaves are ovate, 7-15 centimeter long, with bluntly serrated edges and a distinct point at the end; the leaf margin is revolute, the very edge being curled under, a diagnostic character which distinguishes it from all other alders. The leaves turn yellow in the autumn before falling. The male flowers are dangling reddish catkins 10-15 centimeter long in early spring, and female flowers are erect catkins which develop into small, woody, superficially cone-like oval dry fruit 2-3 centimeter long. The flowers are wind pollinated, and a large amount of airborne pollen is produced in early spring. The seeds develop between the woody leaves of the 'cones' and are shed in the autumn and winter.

Surface roots can be problematic if the tree receives frequent, shallow watering. Better to water deep and infrequently. Established trees with deep roots can be surprisingly drought tolerant. This is a large tree that requires plenty of space. Best suited for restoration or large gardens with bioswales or streams.
Plant Description
Plant Type

Max. Height
50 - 98.4 ft (15.2 - 30 m)

Max. Width
45 ft (13.7 m)


Growth Rate

Winter Deciduous

Flower Color
Red, Yellow

Flowering Season
Spring, Winter

Native Status

Natural Setting
Site Type
Wet open places such as river or stream banks, pond edges, springs, and bioswales

Shade, Part Shade

Elevation ?
2' - 6415'

Annual Precip. ?
11.9" - 126.2"

Summer Precip. ?
0.16" - 4.19"

Coldest Month ?
29.3° F - 50.4° F

Hottest Month ?
56.8° F - 78.8° F

Humidity ?
0.01 vpd - 26.57 vpd

Soil Description
Tolerates a variety of soils as long as adequate moisture is available

Soil PH
4.0 - 7.0

Soil Toxicity Tolerance
Tolerates Saline Soil


Cold Tolerance(° F)
Tolerates cold to 15° F

Sunset Zones ?
2, 3, 4*, 5*, 6*, 7*, 14, 15*, 16*, 17*, 20, 21, 22, 23, 24

Companion Plants
Use with other wet-area trees and shrubs such as Cottonwood (Populus sp.), Willow (Salix sp.), Sycamore (Platanus recemosa), Monkeyflower (Mimulus cardinalis or guttatus), Yerba Mansa (Anemopsis californica), wild Rose (Rosa californica), Hedge Nettle (Stachys bullata), and native ferns.

Landscaping Information
Ease of Care
Very Easy

Water Requirement ?
Moderate - High
Extremely Low
Very Low
Moderate - High

Rarely Used

Max. Summer Water ?
No Summer Water
Keep moist

This tree is subject to infestation by mistletoe. Infected branches should be pruned out.

Pest Control
This tree is subject to infestation by mistletoe.

Common uses
Bird Gardens, Butterfly Gardens

Nursery Availability
Commonly Available

Sources include: Wikipedia. All text shown in the "About" section of these pages is available under the Creative Commons Attribution-ShareAlike License. Plant observation data provided by the participants of the California Consortia of Herbaria, Sunset information provided by Jepson Flora Project. Propogation from seed information provided by the Santa Barbara Botanical Garden from "Seed Propagation of Native California Plants" by Dara E. Emery. Sources of plant photos include CalPhotos, Wikimedia Commons, and independent plant photographers who have agreed to share their images with Calscape. Other general sources of information include Calflora, CNPS Manual of Vegetation Online, Jepson Flora Project, Las Pilitas, Theodore Payne, Tree of Life, The Xerces Society, and information provided by CNPS volunteer editors, with special thanks to Don Rideout. Climate data used in creation of plant range maps is from PRISM Climate Group, Oregon State University, using 30 year (1981-2010) annual "normals" at an 800 meter spatial resolution.

Links:   Jepson eFlora Taxon Page  CalPhotos  Wikipedia  Calflora

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