How to Grow and Care for Spiraea Shrubs

When looking for flowering shrubs, people often think about hydrangea, lilac, rhododendron or azalea, and viburnum. With its 80 or so species, the genus Spiraea is quickly rising up to become a garden favorite. The species are easy to grow and maintain and there is a wide range of readily available cultivars and hybrids on the market.  

The Spiraea genus as a whole has a few distinct identifying physical features. Each is deciduous, mostly with narrow, oval-shaped leaves called lanceolate. They all have many tightly clustered flowers with five sepals and five petals. These similarities are often as far as it goes when it comes to the genus. This is great news for designers and gardeners looking for variety in their landscapes.

Spiraea Care

Choosing an area that drains well and gets a fair amount of sun is essential for all spiraea species. It's still important, however, to do your research when searching for a suitable species for your garden, as their care and condition requirements can vary considerably. The details below are general guidelines and not specific to any particular species.

Light

To achieve the best bloom production and foliage coloration, ensure that the shrub gets full sun.

Soil

Spiraea prefers well-drained soil that has a neutral pH. Test the acidity level before planting and occasionally during the season to determine if soil amendments are needed. 

Water

Keep newly planted Spiraea well-watered until they have established themselves. After this point, it is okay to taper off watering as it is only necessary when the soil is dry. This method is preferred because the genus has a tendency to develop root rot and overwatering can cause serious issues.

Temperature and Humidity

Spiraea can be winter hardy down to 15 degrees Fahrenheit. It is a good idea to check the individual species' details, but the general range is USDA 4-8, with some being a little bit pickier than others.

Fertilizer

The Spiraea genus is not one that feeds heavily. If it is producing healthy and vigorous blooms, no fertilization may be needed. If feeding is required, it should only be done annually in the early spring with a slow-release fertilizer. 

Types of Spiraea

There are many different Spiraea species to choose from. Below are just a few popular choices available for growing in North America.

• Birchleaf spiraea (Spiraea betulifolia): While it has some summer interest in small white flowers, the real highlight is the show this shrub puts on during the fall with purple, orange, and red shades.
• Early Spiraea (Spiraea thunbergii): Blooming early in the spring, this species offers three-season interest giving the most spectacular fall colors of purple, orange, burgundy, red and yellow.
• Japanese spiraea (Spiraea japonica): This species is easily the most cultivated with endless sizes, forms, colors, and textures. One popular cultivar is 'Gold Mound'.
• Bridal wreath spiraea (Spiraea prunifolia): Bridal wreath spiraea is a large species compared to others in the genus. It can reach heights up to 20 feet if not pruned back. Bridal wreath spiraea is named for the beautiful wedding wreaths traditionally made from its tiny white flowers. During autumn, reds, oranges, and yellows pop out of nowhere, setting the large shrub ablaze. 

Pruning

Any spiraea plant can benefit from a vigorous pruning of old wood. Cut each stem back, leaving at least five buds on each. Thin out branches from the middle to improve overall air circulation around the plant. Remove any suckers, and generally tidy up the plant. Most species should be pruned only after flowering is finished for the season.

Propagating Spiraea

There are three reliable ways to propagate spiraea: softwood cuttings, hardwood cuttings, or ground layering.

When taking a softwood cutting, cut healthy stems that are 10 to 12 inches in length. Remove the bottom leaves from the stem, dip the cutting in rooting hormone, and plant four or five of the cuttings in a 6-inch pot that has been filled with moist yet well-drained soil. Place a plastic bag over the cuttings and put them in a location with dappled sunlight, checking periodically to make sure the soil is moist. New growth should appear in about a month.

When taking a hardwood cutting, do it only in the winter, during a period of dormancy. Trim both the bottom and the top of the stem just above a leaf node. Place the cutting in soil at a depth of about 4 to 6 inches. Keep the soil moist and look for new growth from the cuttings in the spring.

Finally, to propagate spiraea by ground layering, choose a long stem that is flexible enough to lie flat on the ground. Keep it attached to the mother plant. Remove the leaves and scrape away at the stem to open it up a bit. Do this for only about 3 inches of the stem that will be in contact with the ground. Gently bend the stem until the scraped area touches a small trench of about 1-inch deep that you have dug into the ground right below it. Pin the stem to the ground to hold it steady. Cover it with soil, carefully water it, and give it a few months to establish roots. Once it has, you can remove it from the area and transplant it to a new location.

How to Grow Spiraea From Seed

Germinate spiraea seeds during the early spring by placing the seeds in a damp paper towel in a dark area until they sprout. Once they do, plant them in 12-inch pots, taking care to follow the instructions on the seed packet for your particular variety of spiraea, as the growing requirements might differ. After planting, cover the pot with plastic wrap and set it outside in the sun. The seedlings should sprout well within a matter of weeks. Plan to keep the spiraea in the container for a year to allow them to grow strong enough to handle the forces of nature.

Potting and Repotting Spirea

Follow the usual guidelines for planting a shrub and dig a hole two times as wide as the container or rootball, and just as deep. Refill the hole and cover with a 2- to 3-inch layer of mulch out to the drip line. Be sure to keep the mulch away from the stems to avoid damaging the shrub. If planning on a grouping, be mindful of the size at maturity and plant accordingly.

Overwintering

Established spiraea shrubs can usually handle winter just fine. However, to help them along, water the soil deeply in the week before the first frost hits and give them a good layer of mulch after the temperatures drop.

Common Pests and Plant Diseases

Aphids and spider mites are two common pests that you might find on your spiraea shrubs. These can be remedied with a good blast of water or horticultural oil. The plants might also fall victim to powdery mildew, especially if they are in an area where they don't receive enough sunlight. Fungicides can help with that problem.

How to Get Spirea to Bloom

If your spiraea isn't blooming, keep in mind that it might take a season or two for a new plant to establish a good root system. If that's not the problem, then look at the potential for overwatering, which can be fatal for the plant. Space the shrubs appropriately so that airflow is not an issue, fertilize very lightly once a year, and deadhead any flowers you do have to encourage new blooms.