How to Grow and Care for Sweet Pea Shrub

Sweat pea shrub with purple, red and pink flowers on thin stems in garden

The Spruce / Heidi Kolsky

Sweet pea shrub (Polygala × dalmaisiana) is a leggy, fast-growing broadleaf evergreen shrub with beautiful flowers. It is thought to be a hybrid between two South African species (P. opposistifolia and P. myrtifolia), and is sometimes known as Polygala myrtifolia grandiflora. Sweet pea shrub's lovely gray foliage remains evergreen down to about 40 degrees Fahrenheit, and in regions that are never touched by frost, it may bloom nearly year-round with purple-pink flowers that resemble those of the sweet pea plant. It can survive in climates with temps down into the low 20s, where it will have a shorter but still impressive bloom period from midsummer into fall. Sweet pea shrub is a fast-growing plant that achieves full size in a single growing season when planted from a potted nursery specimen. It is normally planted in the spring after the soil has fully warmed.

Unlike sweet pea plant (Lathyrus odoratus), which is mildly toxic, sweet pea shrub is not toxic to humans or to pets.

Common Name Sweet pea shrub (bush), milkwort
Botanical Name Polygala × dalmaisiana, Polygala myrtifolia var. grandiflora
Family Polygalaceae
Plant Type Broadleaf evergreen shrub
Mature Size 3–5 ft. tall and wide
Sun Exposure Full, partial
Soil Type Rich, well-drained
Soil pH Acid to neutral (6.0 to 7.5)
Bloom Time Mid-summer through to fall; longer in frost-free climates
Flower Color Purple-pink
Hardiness Zones 9–11 (USDA)
Native Area South Africa

Sweet Pea Shrub Care

Provided your sweet pea shrub gets enough sun and warm weather, it is usually quite easy to grow and maintain. The main challenge is giving the plant the right amount of water at the right time, and pruning it regularly so the plant remains dense and full of blossoms.

Sweet pea shrub is most commonly planted in the spring from potted nursery plants. Prepare the planting area with plenty of organic material, then plant the shrub at the same height it was in its nursery container. Water regularly as the plant is becoming established, and feed it every spring.

Sweat pea shrub with pink flowers on end of stem closeup

The Spruce / Heidi Kolsky

Sweat pea shrub pods on end of stem closeup

The Spruce / Heidi Kolsky

Sweat pea shrub stem with blooming flower with dark red petals closeup

The Spruce / Heidi Kolsky

Sweat pea shrub with purple flowers in garden

The Spruce / Heidi Kolsky

Light

Full sun or lightly dappled positions are best for the sweet pea shrub. This will ensure they flower the most abundantly. If they get too much shade, sweet pea shrub can become overly leggy, with fewer blossoms.

Soil

Sweet pea shrub can tolerate a wide variety of soil types, but its preference is for a well-drained, fertile soil that is rich in organic matter. It doesn't however, appreciate an overly-wet and heavy soil. A slightly acidic pH is preferable, though sweet pea shrub will also tolerate neutral soil.

Water

While a sweet pea shrub is establishing itself, it will appreciate regular watering, especially through the summer months. You should make sure the soil doesn't dry out. For best results, give it about 1 inch of water each week, but don't irrigate on weeks where your garden has received ample rainfall. Once established, sweet pea shrub is surprisingly tolerant of short droughts, such as if it misses water for two or three weeks during a summer vacation. But extended droughts that completely dry out the plant's roots can be deadly.

For this reason, it's a good idea to add a thick mulch around the base of the plant once it has established. This will help it to retain moisture, even when it only receives infrequent watering. Mulching will also help protect the shrub as the winter temperatures arrive.

Temperature and Humidity

For best results, sweet pea shrub should be grown in a warm, frost-free climate (zones 9 to 11). It can, however, still survive when temperatures drop as low as 20 degrees Fahrenheit for a short period. But this plant doe not like prolonged chilly, damp conditions, which cause unpredictable, sporadic growth.

Fertilizer

Using a balanced slow-release fertilizer in the spring can encourage strong blooming. In regions with especially long blooming periods, a second feeding in the fall is often helpful. Container plants may require monthly light feeding.

Types of Sweet Pea Shrub

There are no named cultivars of sweet pea shrub commonly sold. However, there is a related plant, Polygala fruticosa, which is often mistaken for P. x dalmaisiana. P. fruticosa goes by the common name dwarf sweet pea shrub, and it has a number of named cultivars, including 'Petite Butterfly', 'Africana', and 'Southern Shore'. Dwarf sweet pea shrub is very similar to its larger cousin and has nearly identical care needs, but it grows to only about 3 feet in height and spread. Because it is more compact, dwarf sweet pea shrub can be the better choice for growing in containers.

Make sure to check plant labels to make sure which of the two species you are buying, as the common names are sometimes used interchangeably.

Pruning

Because this shrub can become rather leggy, pruning in early spring will help to ensure it keeps a compact and tidy shape. Annual pruning to eliminate sparsely blooming lower stems will encourage more abundant flowering. To avoid sacrificing blooms, try to do your pruning after the main flowering periods have concluded.

Propagating Sweet Pea Shrub

Sweet pea shrubs are best propagated from softwood cuttings at the start of the summer. Here's how to do it:

  1. Use sharp pruners to cut a 4- to 6-inch length of stem containing at least two sets of leaves—and preferably no flowers. If possible, make the cut just below a leaf bud.
  2. Remove the lower leaves on the cutting, leaving at least one set of leaves at the top.
  3. Dip the snipped end of the cutting in rooting hormone, then plant it in a pot filled with standard potting mix. Dampen the potting mix and tamp it down firmly around the base of the cutting.
  4. Place the potted cutting in a loosely secure plastic bag to hold in moisture, then place the pot in a location with plenty of indirect light (not direct sunlight), at a temperature of about 70 degrees Fahrenheit.
  5. Check the cutting periodically to see if roots are developing (tug gently on the stem; when you feel resistance it means the cutting is rooted).
  6. When the cutting has developed roots and is beginning to send out new leaves, remove the pot from the plastic bag and continue to grow it in a sunny location. Keep the potting mix well watered as the cutting is developing into a viable plant.
  7. After a few weeks of growing in the pot, your sweet pea shrub can be transplanted into a larger pot or into its permanent garden location.

How to Grow Sweet Pea Shrub From Seed

Standard sweet pea shrub is a hybrid that does not "come true" if you plant the seeds it produces. However, there are similar species in the Polygala genus—P. fruticosa (dwarf sweet pea shrub), for example—that can be readily propagated by seed under the right conditions. Spring temperatures above 60 degrees Fahrenheit, plenty of sunlight, and consistently moist soil are preferable if you want to direct-sow seeds into the garden.

Or, you can start seeds indoors in small pots or seed trays filled with potting mix. Plant the seeds so they are barely covered, about 1/16 inch deep. Moisten the soil, then cover the tray with plastic and place it in a warm area with plenty of bright indirect light. Keep the potting mix moist, but not wet, during the germination period.

After the seeds germinate and sprout, uncover the pot or seed tray and place it in direct sunlight to grow the seedlings into viable plants. If using seed trays, wait until the plants have at least two sets of true leaves before transplanting them into their own pots.

When outdoor daytime temperatures are reliably at 70 degrees Fahrenheit or above and nighttime temperatures are falling no lower than 40 degrees, the plants can be moved to permanent garden locations. Before outdoor planting, it's best to harden off seedlings by giving them increasingly long daily visits to outdoor conditions for a week or two.

Potting and Repotting Sweet Pea Shrub

Though standard sweet pea shrubs are a fairly large plants, it is not uncommon to grow them in sizable patio containers. The plant is amenable to hard pruning, which allows it to remain nicely trained as a container plant. Ordinary commercial potting soil works well as a growing medium for containers. Use the largest pot that is practical, which will limit how often you must repot. Make sure the container drains well.

Watering and feeding must be done more frequently with container-grown shrubs. In winter, your container sweet pea shrub should be moved into a sheltered outdoor location. It is usually not practical to overwinter a standard sweet pea shrub indoors, but it is sometimes done with the dwarf variety, P. fruticosa.

Overwintering

In most cases, no winter protection is needed for these plants, but in the northern parts of zone 9, where winter temps might be expected to drop to 20 degrees Fahrenheit or lower, a thick layer of mulch helps guard frost damage to the roots. In colder climates, these plants may need to be sheltered against cold winter winds in order to survive.

Common Pests

This plant is free of serious disease problems, and the only common pests are whiteflies and aphids, both of which are easily controlled with horticultural oils or chemical pesticides.

How to Get Sweet Pea Shrub to Bloom

This plant will flower more robustly in full sun locations. And regular hard pruning will keep the plant dense and full of flowers. Like many plants with long bloom periods, sweet pea shrub will bloom best if it gets a spring feeding with balanced fertilizer. In zones where the plant blooms into fall, a second feeding is recommended.

Common Problems With Sweet Pea Shrub

Other than mild insect issues, this plant registers only a couple of common complaints:

Shrub Is Too Leggy and Sparse

With age, a sweet pea shrub can become leggy with thick, woody stems that produce few flowers. Hard pruning will often help restore the plant, but a very old sweet pea shrub sometimes needs to be removed and replaced with a younger plant.

Leaves Are Yellow

Yellowing leaves is a symptom of too much water. The correct watering intervals are tricky with this plant, as it craves regular moisture but rebels if it receives too much. Don't irrigate this plant on weeks where your garden has received rainfall. Before watering again, make sure the top 2 inches of soil are dry to the touch.

FAQ
  • How can I use sweet pea bush in the landscape?

    The sweet pea shrub develops into a rounded mound with a loose, relaxed appearance. It is a common plant in cottage gardens and rock gardens. It can also be massed for use in shrub borders or in foundation plantings. Sweet pea shrub eagerly accepts hard pruning, making it also useful for informal hedges. Potted plants can be effective in courtyards and patio gardens.

  • How long does a sweet pea shrub live?

    While there are reports of individual plants living as much as 20 years, it's common for sweet pea shrubs to gradually produce fewer flowers, usually because they aren't pruned regularly. Thus, homeowners sometimes remove still-living mature plants because they are no longer attractive in the landscape.

  • Does sweet pea shrub attract butterflies?

    This plant is not known as a butterfly magnet, but it is decidedly attractive to pollinating bees. Avoid the use of pesticides if you want to be a bee-friendly gardener.

  • Can I rejuvenate a sweet pea shrub?

    Yes. A plant that has become sparse and leggy with few flowers should be cut back so the stems are no more than 10 inches long. The thickest, woodiest stems should be removed entirely. The following year, dense new growth should begin, and within a season you'll once more have a spectacular plant.