How to Grow Blue Blossom Ceanothus

blue blossom ceanothus flowers

Miguel Vieira / Wikimedia Commons / CC BY 2.0

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The blue blossom ceanothus (Ceanothus thyrsiflorus), also known as blue mountain lilac, is an evergreen shrub that is native to the West Coast of North America. The shrub has a rounded form with arching branches that bear glossy green elliptic to ovate leaves. Large clusters of tiny pale to dark blue flowers with yellow stamens appear in the late spring. And they tend to attract bees, butterflies, and other pollinators. The flowers give way to small purple or brown fruits that also are attractive to wildlife. Blue blossom ceanothus has a quick growth rate and can be planted in the fall or spring.

Botanical Name Ceanothus thyrsiflorus
Common Names Blue blossom ceanothus, blue mountain lilac, blueblossum, blue brush
Plant Type Shrub
Mature Size 4–12 ft. tall, 5–6 ft. wide
Sun Exposure Full, partial
Soil Type Well-drained
Soil pH  Neutral, alkaline
Bloom Time Spring
Flower Color Blue
Hardiness Zones 7–10 (USDA)
Native Area North America
Toxicity Nontoxic

Blue Blossom Ceanothus Care

The blue blossom ceanothus is a fairly low-maintenance shrub. The profuse flowers fall off on their own and don’t require deadheading (the removal of spent blooms). And the shrub is generally healthy and does not have any serious issues with pests or diseases, though you should watch out for common garden pests including aphids and scale. In addition, the shrub is fairly tolerant of drought, salt spray, poor soils, and erosion. 

The main upkeep tasks are typically watering young blue blossom ceanothus shrubs to help them become established, along with pruning as needed. Other than that, the shrub makes an easy-to-care-for addition to the landscape, and it can work especially well as a hedge or on a slope. An important factor to note is this shrub does not take well to its roots being disturbed, so avoid digging around it once it is planted.

Light

The blue blossom ceanothus shrub prefers to grow in full sun to partial shade, meaning at least roughly four hours of direct sunlight on most days. In the warmer parts of its growing zones it prefers more shade, especially from strong afternoon sun. And in the cooler parts of its growing zones it can handle full sun.

Soil

This shrub can grow in a variety of soil types, including loamy, sandy, rocky, and clay soils. The soil ideally should have good drainage, as soggy soil can result in fungal diseases for the shrub. A neutral to slightly alkaline soil pH is best.

Water

The shrub has low to moderate moisture needs, and it has good drought tolerance once it is established. Water young shrubs for their first year or two to ensure that the soil remains evenly moist (but not soggy). After that, you likely won’t have to water your shrub unless your area experiences a prolonged period of drought and/or extremely hot weather that wilts the foliage. Overwatering can lead to root rot and other diseases.

Temperature and Humidity

The blue blossom ceanothus shrub has fairly good heat and cold tolerance. It can handle temperatures down to 15 degrees Fahrenheit. However, in the colder parts of its growing zones it ideally should be planted in a spot that is sheltered from strong, chilling winds. Moreover, humidity typically isn’t an issue as long as there is good air circulation around the shrub. Otherwise high humidity can create prime conditions for fungal growth.

Fertilizer

Fertilization typically is not necessary for the blue blossom ceanothus. At the time of planting, you can mix some compost into the soil to give your shrub a boost and improve soil drainage. Adding a light layer of dry, shredded leaves around the shrub each fall also can continue to benefit the soil 

Blue Blossom Ceanothus Varieties

There are several varieties of the blue blossom ceanothus shrub, including:

  • Ceanothus thyrsiflorus 'Skylark': This cultivar has a particularly long flowering period, and it is more tolerant of slow-draining soils and cold temperatures than the main species plant.
  • Ceanothus thyrsiflorus 'Snow Flurry': This cultivar can grow quite large, and it features glossy dark green leaves with fluffy clusters of white flowers. 
  • Ceanothus thyrsiflorus 'Repens Victoria': This shrub has a mounding growth habit and sports powder blue flowers. 
  • Ceanothus thyrsiflorus 'El Dorado': This cultivar is known for its variegated foliage that is green with golden edges, and it also bears powder blue flowers. 
  • Ceanothus thyrsiflorus 'Arroyo de la Cruz': This cultivar has blue flowers, dense branches, and small leaves, and it is known for its fast growth rate.

Pruning

The blue blossom ceanothus shrub does not need much pruning even once it's mature. And in fact, heavy pruning can actually weaken the shrub’s integrity and harm its health. However, you may prune just the tips of branches to promote a more compact shape. It also can be beneficial to prune out leafless interior branches to encourage new growth and improve air circulation around the shrub. The best time to prune is immediately after the shrub is done flowering for the season.