How to Plant and Grow Penstemon

With a rainbow of colors to choose from, penstemons are lovely additions to the garden. The diversity of these plants is remarkable, with several hundred species available. Penstemons are tough perennials that stand up to intense growing conditions. From dwarf alpine plants to prairie penstemons reaching up to 5 feet tall, there's a height for every part of the garden.

With such a wide variety of species and cultivars to choose from, you can have penstemon blooming for almost the entire growing season. The blooms look stunning in a garden setting and make a wonderful addition to a cut flower arrangement. Along with their bright blooms, many varieties of penstemon also have colorful foliage to add to the mix.

Where to Plant Penstemon

With so many penstemon species available, there are selections suitable for any region within USDA zones 3–9. The tiny alpine varieties, native to North America, can grow in the cracks of rocks and stand up to extremely cold weather; other types grow in disturbed soils in prairie settings.

In general, penstemon plants grow best in infertile, well-draining sandy or gravelly soil. They don't tolerate clay soil or rich soil.

How and When to Plant Penstemon

Sow penstemon seeds outdoors in fall or winter. Choose a location with well-draining, not rich, soil that receives full sun during the growing season and cover them with no more than a 1/4 inch of soil. The seeds require a period of cold weather, called stratification, before germination. Plants grown from seed may not bloom the first season.

A quicker method to acquire flowering penstemon is to buy nursery plants. In spring, plant them shallowly in the garden, spacing them 12 inches apart and making sure the top of the root crown is slightly above the soil line.

Penstemon Care Tips

Light

Penstemon plants need full sunlight to perform their best. Not only does this ensure that they put out quality blooms, but it also prevents them from flopping and reduces the risk of the plant developing powdery mildew on the leaves, especially in humid summers.

Soil and Water

No matter where they grow, penstemon plants need extremely well-drained soil and do not tolerate wet soil, especially over the winter. The prairie-type penstemons are more tolerant of standard garden soils, whereas the alpine varieties require excellent drainage, similar to what is found in gravel gardens and trough gardens. No matter what type of penstemon you plant, they all perform well in fairly nutrient-poor soils and tend to put out too much tender growth in rich soil.

Temperature and Humidity

Most penstemons are cold-hardy to about 24°F. In summer, they tolerate temperatures into the 90s. They grow in a range of humidity levels, depending on the variety.

Fertilizer

Don't give penstemons compost or aged manure; it is too rich for the plants. Instead, apply an organic fertilizer only once a year, according to manufacturer's instructions.

Pruning

To winterize the plant, cut all the foliage to about 2 inches above the soil line in late fall or early winter. During the growing season, remove individual stalks that have flowered at the soil line unless you want the plant to self-seed with the copious amount of seed it produces.

Pests and Problems

Penstemon plants attract spider mites, pests that can be controlled with a strong spray of water or insecticidal soap or neem oil.

A too-damp environment can lead to botrytis leaf mold or powdery mildew. Provide good air circulation around the plants and don't water the plants from the top.

A microscopic nematode called an eelworm can inhabit the soil around the roots of the plant. Litte can be done for the plant at this point; it is best to remove and destroy it. Before planting new plants, replace the soil in the area where the infested plant grew to prevent reinfection.

How to Propagate Penstemon

Penstemon can be propagated by division, stem cuttings, and seed.

To propagate by division: Dig up a mature penstemon in early spring. Cut apart the root system, making sure that each division has a portion of the roots. Plant the divisions back in the garden and water them or plant them in containers to give away.

To increase your plants with stem cuttings: To take cuttings, cut a 5-inch section from the tip of a healthy penstemon plant just below a leaf node. Remove the leaves from the lower half of the cutting. Dip the bottom of the cutting in rooting hormone and insert it into planting medium containing sand and perlite. Cover the pot with a plastic bag that has holes poked in it to combat condensation, and keep the cutting in filtered light in a warm environment. Lift the bag daily, misting if the planting medium is dry. After three or four weeks, gently lift the cutting to see if roots have developed (they should be at least 1/2 inch). If so, repot in a slightly moist potting mixture. If not, return them to the pot and check again in a couple of weeks.

To grow from seed: If your penstemon isn't a hybrid, you can harvest seed from a mature plant approximately six weeks after it blooms. Cut a flower stalk several inches beneath a seed head and place it in a bag to dry for at least a week. After it is dry, pinch the seed head to release the seeds. Plant them outdoors in the fall, as you would with purchased seeds, for plants the following season.

Types of Penstemon

New penstemon varieties are being produced constantly. In many cases, new varieties tend to be improvements of older types, with larger blooms, more compact habits, or darker foliage. There is also work to create continuous-blooming annuals. There are already several on the market that boast extremely long blooming times and a few that bloom nonstop from spring until frost.

Common Penstemon

Penstemon barbatus blooms with narrow spikes of two-lipped tubular flowers about 2 inches long in spring. The colors can be red, pink, carmine, or purple, and the 8-inch lance-shaped leaves have a whitish bloom. The plant can reach 3-5 feet tall. USDA zones 4-9

'Dark Towers' Penstemon

A darker variety of the much loved 'Husker Red' varietal, 'Dark Towers' holds onto its burgundy foliage throughout the summer, and its blooms have more of a pink flush as well. USDA zones 3-8

Penstemon Hirsutus 'Pygmaeus'

Lavender blooms with white lips nod above dense green leaves on this dwarf plant that reaches up to 6 inches tall. USDA zones 3-9

'Electric Blue' Penstemon

Penstemon heterophyllus 'Electric Blue', just as its name implies, has flowers that glow with color. Notably drought-tolerant, this plant can grow up to 18 inches tall. USDA zones 6-9

'Husker Red' Penstemon

Penstemon digitalis 'Husker Red' blooms in early to midsummer with loose spires of 1-inch tubular pink-flushed white flowers. The lance-shaped leaves are deep maroon in color, 4-5 inches long, and arranged in a basal rosette. It grows about 2-1/2 feet tall. USDA zones 3-8

Pine-Leaf Penstemon

Penstemon pinifolius is a summer bloomer with loose spires of 1-inch narrow scarlet tubular flowers. It has needlelike foliage and is slightly woody at the base, making the species an excellent choice for dry rock gardens or walls. USDA zones 4-10

Penstemon Companion Plants

Lady's Mantle

Lady's mantle looks great in the garden and in a vase. Its scalloped leaves catch rain or dewdrops, making them look dusted with jewels, while the chartreuse flowers appear in playful, frothy clusters above the foliage. Lady's mantle is ideal for softening the edge of a shaded path or creating a groundcover in dappled shade.

Lamb's Ear

Lamb's-ears is a top pick for a groundcover in a hot, baked spot. Its silver felted foliage quickly forms a dense, delightful mat while contrasting nicely with other foliage and flowers nearby. Depending on the type of varietal you choose and your growing conditions, your lamb's ear may self-sow freely to the point of becoming a bother. In hot, humid climates, lamb's ears may "melt down" in summer, becoming brown and limp.

Catmint

Catmint is one of the toughest perennials you can grow. It's a proven performer during hot, dry weather, and the silvery foliage and blue flowers look great most of the season. Deadhead or cut back hard after the first flush of bloom to encourage more flowers. Average, well-drained soil is usually sufficient. Tall types may need gentle staking; it sometimes seeds freely.