Canadanthus modestus (Giant Mountain Aster)

Canadanthus spp.

great northern aster

Asteraceae Northern North America

Attracts Pollinators Attracts Butterflies

Native to North America

SunFull Sun – Part Shade

WaterMedium

HardinessZones 2–6

Overview

Canadanthus spp. is a genus in the aster family (Asteraceae) made up of the great northern aster, Canadanthus modestus, a perennial wildflower of cool northern climates. Once classed within Aster spp., it forms upright, glandular-hairy stems 8-40 inches (20-100 cm) tall that spread slowly by rhizomes into loose colonies 1-2 feet (30-60 cm) wide. The lance-shaped leaves are 2-5 inches (5-13 cm) long, toothed or smooth, and clasp the stem at their bases. From mid to late summer the stems carry flower heads 1-1.5 inches (2.5-4 cm) across, each with 15 to 35 narrow purple to violet rays around a yellow central disk. The whole plant is covered in sticky glandular hairs that set it apart from many similar asters. Bloom comes late in the season, supplying nectar and pollen when few other plants flower in its range. Growth follows moist, cool soil in open woods, thickets, clearings, and streambanks across the boreal zone. In warm or dry gardens the plant struggles, and its rhizomes can spread beyond where they are wanted in rich, damp ground.

Native Range

Canadanthus modestus is native to northern North America, from Alaska and across boreal Canada south through the Great Lakes region and along the Rocky Mountains to the northwestern United States. It grows in moist woodland openings, thickets, and along streams, often in cool mountain or subalpine sites.

Suggested Uses

Plants are used in cool native-plant gardens, moist woodland borders, and rain gardens, and in naturalized plantings across northern regions. The late flowers fit pollinator plantings that aim to carry nectar into autumn. Suited to northern or upland gardens, the genus performs poorly in hot summer climates.

How to Identify

Sticky glandular hairs cover the stems, leaf surfaces, and flower bracts, giving the plant a clammy feel that marks it among northern asters. Flower heads are 1-1.5 inches (2.5-4 cm) wide with 15 to 35 thin purple to violet rays and a yellow disk. The lance-shaped leaves clasp the stem with rounded or slightly lobed bases and have toothed or smooth margins.

Appearance

Size & Dimensions

Height8" - 3'4"

Width/Spread1' - 3'

Reaches mature size in approximately 3 years

Colors

Flower Colors

Foliage Colors

Fall Foliage Colors

Bloom Information

Bloom Period

~6 weeks

Summer Fall

Flowering occurs from July to September, later than most asters in the same range. Heads open over several weeks, with the violet rays fading as the central disk turns reddish-brown. The late bloom carries nectar into early autumn before frost ends the season.

Detailed Descriptions

Flower Description

purple to violet with yellow center

Foliage Description

green

Growing Conditions

Sun Requirements

SunFull Sun SunPartial Shade

Requires 4-8 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range5.5 - 7.0(Neutral)

357912

Soil Types

loam silt

Drainagemoist

Water & Climate

Water Needs

Medium

Frost Tolerance

hardy

Time to Maturity

2-3 years from seed

Care & Maintenance

Care Guide

Plants grow in moist, humus-rich soil that stays cool through the season, with a pH around 5.5 to 7.0. Part sun to light shade suits the genus, matching the woodland edges and clearings of its native range. Steady moisture is needed, as the shallow roots dry out fast in heat. Cold is not a concern, since the plant is hardy into the boreal zone, but prolonged summer heat shortens its life. Spread by rhizomes is slow but steady, and clumps can be divided in spring to renew them. Few pests trouble the foliage, though powdery mildew can appear in crowded, still conditions.

Pruning

Little pruning is required. Stems can be cut to the ground after frost, or left standing through winter so the seed heads feed birds. Cutting plants back by half in early summer holds them lower and reduces flopping, at the cost of a later bloom.