How to grow Penstemon
Also known as beardtongue, penstemon is a North American relative of other garden favourites, foxglove and snapdragon. There are hundreds of species, including some alpine types, though by far the most popular are those known as ‘border penstemons’. These produce spikes of tubular flowers, generally blooming from midsummer right up until the first frosts – long after most other plants have given up in exhaustion. There are a vast number of cultivars available offering a huge range of colours. Some are hardier than others, therefore it always pays to check the individual description to ensure you choose the right plant for your garden conditions. As a general rule, the thinner the leaves the hardier the penstemon. Those with fatter, broader leaves tend to be more on the tender side.
If this makes penstemons sound tricky, they really aren’t. Grown in the right conditions, these workhorse plants thrive with the minimum of fuss, providing continuous colour for many months on end. Their extremely strong stems stand up to all but the strongest of winds, meaning staking is only required in very exposed gardens.
Key Information
Soil pH
Position
Hardiness
Where & when to plant Penstemon
Position - A sunny, sheltered spot is ideal (with at least half a day of full sun each day during spring and summer). Penstemon will thank you for plenty of elbow room, i.e., not grown too close to other plants.
Soil - Any fertile, well-draining soil. Poorer soil is fine, though does mean more watering and feeding will be required. Avoid waterlogged soil.
Flowering Period - Usually midsummer through to late autumn
Hardiness - Hardy, though to what extent varies according to species and cultivar (check before buying and choose according to your local conditions).
The majority are rated H4 (-5 to -10°C), i.e., fine in the average UK winter.
Even hardy penstemon (those rated H4 to H7, i.e, the majority of border penstemons) can struggle with a UK winter when young and newly planted. The best results are therefore achieved from a spring planting, to allow plenty of settling in time before heavy rain and frosts hit. Plant from March to mid-May as the soil begins to warm up (look out for any weed seeds germinating: nature’s cue that the time is right).
Planting can also be carried out in summer, though be prepared to water regularly. Autumn is an option if you have a mild microclimate and can offer very well-draining soil.
In terms of location, penstemon is one of the best plants to weave through beds and borders, filling gaps between other perennials. It can also be grown successfully in a container.
The handful of penstemon rated H3 (half-hardy), should be planted outside only if you can offer a warm, sheltered spot (such as at the foot of a south-facing wall or an urban courtyard garden), or are prepared to offer winter protection – see ‘Cold Protection’ below. Otherwise, grow these in containers and overwinter under cover.
How to plant Penstemon
· For planting in the garden, dig the soil area removing any large stones and weeds and breaking up any lumps. Penstemon is a hungry plant (it takes plenty of nutrients to flower so long and hard), so take this chance to incorporate a generous amount of organic matter. Manure, leaf mould, or garden compost all work well. Rake level and firm with your heels. Rake level again.
· Water plant well and allow to drain before planting.
· Dig a hole twice the width of the root-ball.
· Place the plant in the hole, ensuring the top of the root ball sits level with the surface of the soil. Too low and the plant may rot, too high and the roots can dry out.
· Backfill with soil and firm in gently with your foot.
· Soak well with water.
· Mulch around the base with well-rotted organic matter.
· If you’re planting multiples, aim for a spacing of around 45 centimetres.
· For planting in a container, find a pot a few centimetres larger than the rootball, ensuring there are plenty of drainage holes.
· If you are using a heavy pot, it can be a good idea to fill and plant it in situ to save yourself the trouble of moving once full.
· Use a good quality potting compost with plenty of horticultural grit mixed in, and, if not already present in the compost (check the description on the bag) some slow-release fertiliser granules.
· Start by partially filling the pot with compost; enough so that when placed on it the upper surface of the root ball is about 3cm lower than the top of the pot.
· Infill all the space surrounding the root ball with compost, firming down with your fingers then adding a little more so the plant is held tight.
· Pick up the container and lightly tap on the potting bench or ground a few times to help further settle the compost around the plant.
· Soak well with water.
· A mulch with horticultural grit will look attractive and help to prevent a ‘cap’ or crust forming on the top of the compost (something container plants can suffer due to the artificial nature of their watering).
What to plant with Penstemon
Penstemon are available in a glorious range of soft pastel and bright jewel colours and fit well into both contemporary and traditional cottage planting schemes adding a mid-height level to the garden border.
Combine this cottage garden favourite with other easy, long-flowering classics such as geranium, alchemilla, achillea, erigeron, antirrhinum, nepeta, roses and erysimum. A hassle-free, floriferous display is yours for the taking!
How to care for Penstemon
Pruning and Deadheading
Penstemon is a relatively short-lived perennial, tending to become woody and less floriferous after about five years (we recommend taking cuttings to ensure replacements waiting in the wings. See below). However, getting into a simple pruning routine can help to extend this lifespan:
· In autumn, cut plants back by a third. This reduces the chances of wind rock, while leaving enough foliage on the plant to protect the crown and roots over winter. Then…
· In April/ May, cut each stem down to its lowest shoots.
Throughout the growing season, deadheading will help maximise flowering. Cut faded spikes just below the lowest flowerhead to encourage further blooming.
Watering
Penstemon in the ground should be given a good watering in on planting, followed by a layer of mulch to lock the moisture in (see next section for more on this). Soak a couple more times over the coming weeks, after which penstemon is fairly drought tolerant and should need watering only in prolonged periods of very hot, dry weather. The exception to this advice is if you’re growing yours in very free-draining soil, in which case watering will be needed more frequently.
With a more limited root run, penstemon grown in a container requires regular watering throughout the growing season. To avoid overdoing it, a good rule of thumb is to allow the top couple of centimetres to dry out between soakings. To check this, wiggle your finger down into the compost until you feel moisture. Be aware that in the height of summer containers can need watering every day, particularly those in full sun.
Feeding
It takes plenty of nutrients to power such impressive flowering, and as such penstemon benefits from a little attention when it comes to feeding. Get off to a flying start by improving the soil during the planting process, then applying an annual mulch of well-rotted organic matter (this has the added benefit of suppressing weeds and locking in moisture). The best time to mulch is autumn or spring.
A spring application of balanced feed (such as blood, fish, and bone) can also be a good idea. Repeat in midsummer if you garden on poor or free-draining soil.
Container-grown plants rely even more on the gardener for nutrition. The first step is to make sure you use a good quality compost with slow-release granules mixed in. These generally provide nutrients for around 6 to 8 weeks, after which you’ll need to apply a balanced liquid feed every 2-3 weeks until the end of the growing season (i.e., September-ish).
Cold Protection
Even hardy types of penstemon can suffer in the winter – particularly when young. Be sure to plant in well-draining soil (waterlogging is the quickest way to kill this plant off), and don’t prune back fully until mid to late spring. If your winters are harsh, we advise choosing an especially hardy variety such as ‘Dakota Burgundy’.
Like all plants, penstemon can be more susceptible to cold damage when grown in a container. It is worth wrapping the pot in an insulating layer of hessian, fleece, or bubblewrap which, as well as protecting the roots, helps prevent frost damage to the container itself.
Half-hardy (H3) penstemon will need to be protected with a cloche or horticultural fleece whenever a hard frost is forecast. Alternatively, grow in a pot and overwinter somewhere cool yet frost free, such as an unheated greenhouse, conservatory, or porch.
Pests and Diseases
Penstemon is largely trouble free (deer and rabbits notably avoid it), though you may see powdery mildew cropping up from time to time. This fungal disease appears as a white, powder-like coating on foliage, and can be caused by poor air flow, drought stress, or insufficient sunlight.
A little TLC usually rectifies the problem. Simply prune off and dispose of any affected leaves, give the plant a good soak (avoiding water on the foliage if possible), and apply a generous mulch of well-rotted organic matter. It can also be worth thinning out surrounding plants to improve airflow. Do not be tempted to give an extra feed, as this will generate an excess of soft, new growth which is particularly susceptible.
How to propagate Penstemon
It is a good idea to propagate from your penstemon, both as insurance against winter losses (this is especially relevant for the less hardy varieties) and to ensure you have replacements ready when more mature plants start to run out of steam.
While penstemon can be grown from collected seed, the results can be variable and unlikely to come ‘true’ if there is more than one species or cultivar grown in the vicinity. A much more reliable approach is to take cuttings in summer or early autumn. Penstemon is amongst the easiest plants to take cuttings from, so much so it is often recommended as a starting point for children or novice gardeners.
1. In the morning (plants are more turgid at this time and less likely to wilt during the process), take 10-12 centimetre from non-flowering shoots. Snip off just above a bud using sharp, clean secateurs.
2. Place in a clean plastic to transport the cuttings back to a potting bench or garden table.
3. Fill a container with a well-draining compost mix (50% grit or perlite to 50% compost is perfect).
4. Trim the end of the cutting to just below a node (point at which leaves grow).
5. Remove the lowest third of the leaves, and the soft tip too.
6. If the remaining leaves are large, these can be cut back by half to reduce water lost through transpiration.
7. Insert the cuttings into the compost and water lightly. Several cuttings can be put in the same container if there is enough space to do this without them touching.
8. Place in a propagating unit with bottom heat if you have one, or covered with a plastic bag on a warm windowsill if not (out of direct sunlight). If using a plastic bag, remove it a couple of times a week for around ten minutes. This will ensure the cuttings remain well-ventilated and don’t rot.
9. Keep the cuttings misted and occasionally watered until they root. You will know this has happened when roots emerge out of the bottom of the container – expect this to take two to four weeks.
10. Gently remove rooted cuttings and pot them into individual pots. Grow on in a protected environment such as a conservatory, greenhouse, or that windowsill again, before planting out as above the following May.
* Many plants carry Plant Breeders Rights and cannot be propagated for commercial purposes.
Common Penstemon Questions
·
· Does penstemon die back in winter?
Penstemon can be evergreen, semi-evergreen, or deciduous, depending on the species. Most border penstemon are semi-evergreen, meaning they will lose some leaves but retain others. Follow our pruning and cold protection advice above to help your plants emerge from winter in the best state possible.
· Does penstemon self-seed?
If seedheads are left intact for long enough, some species and cultivars may self-seed. Note the offspring may vary from the parent plant if several varieties are grown together.
· Do penstemon flowers make good cut flowers?
Sadly, penstemon flowers last only a couple of days in a vase. They are much better enjoyed on the plant.
