How to Cut Back Penstemon

Pruning during the Growing Season

Cut stems back if your penstemon plant starts looking rough around the edges. While you can just let penstemon grow naturally, the stems may grow uneven and may not look as cosmetically pleasing. Use a pair of gardening sheers and place them up to one-third down from the tip of the stem. Squeeze the handles together to cut the stem at a 45-degree angle so water doesn’t pool on top. Continue trimming any other stems that are longer than the others. Never trim off more than one-third of the growths during the growing season, or else your penstemons may not bloom.

Deadhead the flowers to promote a second round of blooms. Wait until after your penstemon blooms and the flowers begin to wither. Place a pair of pruning shears or a gardening knife just below the set of healthy leaves closest to the bloom. Make your cut at a 45-degree angle to prevent the stems from rotting. Flowers usually appear sporadically after deadheading, so you may not get a full second bloom.

Prune one-third off of each stem's height in the fall to winterize the plant. As soon as the flowers start withering and turning brown, you can prune your penstemons. Hold a pair of pruning shears a third of the way down the stem from the tip. Keep the shears at a 45-degree angle to the stem and make your cut. Work your way around the plant, cutting back all of your stems so they’re only two-thirds their original height. Leave the remaining growth from the summer when you’re pruning, or else you may risk your penstemons dying during the winter.

Trimming Old Growths in Spring

Start trimming your penstemon after the last frost in spring. Leave the remaining growth on your penstemons over winter to protect them from any harsh weather and help them survive. Check online for the last estimated frost date in your area or climate so you know when to start cutting. When the weather starts warming up and there’s no risk for frost, it’s safe to cut the stems. You can find the last expected frost date for your area here: https://www.almanac.com/gardening/frostdates. If you cut back your penstemons any earlier, they may not grow during the next season.

Trim growths that are dead or don’t have new shoots down to the base. Examine your penstemon’s stems to see if they have any fresh green shoots growing from them. If you notice a stem that doesn’t have any shoots, place your prunings shears just above the soil and make your cut. Then check for any stems that are cracked, withered, or damaged and remove them since they won’t produce any healthy shoots. Be sure to check the stems in the middle of the plant as well to help thin it out.

Cut every stem just above the lowest shoots with your pruning shears. Look near the bottom 6 inches (15 cm) of the stems and look for fresh green shoots starting to grow from them. Place your pruning shears at a 45-degree angle just above the node for the lowest shoot on the stem. Snip through the stem to cut it off of the plant. Check the remaining stems on your penstemon and cut them back the same way. You use the stems you cut off as compost to help add more nutrients to your soil.

Remove any weeds growing around your penstemon. Check the soil surrounding your penstemons and make sure there aren’t any other plants within 18 inches (46 cm) of your stems. If there are, pull them by the roots out of the ground so they don’t steal any nutrients from your plant. Mulching or laying a weed barrier around your penstemons can help prevent weeds from sprouting.

Taking Cuttings for Propagation

Cut 4–5 in (10–13 cm) non-flowering tips in the summer. You can take your cuttings at any point during the summer while they’re growing. Look at the ends of the stems and find a few that don’t have any flowers growing from them. Put your pruning shears 4–5 inches (10–13 cm) from the tip of the stem just below one of the nodes with leaves growing from it. Make a 45-degree angle cut to take your cutting. Avoid cutting stems that have flowers since they won’t grow as well.

Pull the bottom leaves off your cuttings. Locate the 2–3 leaves that are closest to the cut end of your cutting. Pinch the base of the leaf and gently pull it from the stem. That way, you expose more of the stem to help promote healthy root growth. You may also trim a third off of the leaves that are higher on your cutting to help reduce moisture loss when you replant them.

Dip the cut ends into a rooting hormone. Rooting hormone helps stimulate the plant’s growth and helps protect the roots from fungus. Pour some rooting hormone into a dish and place the cut ends of the plant in the powder. Coat the exposed node where you removed the leaves as well so they will sprout new roots. Apply the hormone to all of the cuttings you took so they’re more likely to survive. You can buy rooting hormone online or from your local gardening supply store. Throw away any rooting hormone that’s left in the dish rather than putting it back in the container. Otherwise, you could contaminate it and potentially spread diseases harmful to your plants.

Stick the cuttings into a pot with a mix of compost and perlite. Fill a 3 ⁄2 in (8.9 cm) pot with a mix that’s equal parts compost and perlite. Take your cuttings and place the stems into the soil around the edges of the pot. Push the stems down so the lowest leaves are just above the soil’s surface so they’re more likely to grow roots. You can grow up to 5 penstemon cuttings in a single 3 ⁄2 in (8.9 cm) pot. You may also use a modular planting tray if you want to root more cuttings.

Water your cuttings until the potting mix feels damp. Fill a watering can with fresh, clean water and slowly pour it onto the potting mix. Let the water absorb and sink into the pot before adding more. Once you see water running out of the drainage holes at the bottom, stop watering your penstemons. Avoid overwatering your penstemon cuttings since they could develop root rot and will not survive.

Cover the pot with a plastic bag or put it in a propagator. Place 4 bamboo poles that are taller than your cuttings into the potting mix so they form a square. Set a large plastic bag over the bamboo poles to help trap the moisture so they’re more likely to grow. Alternatively, you can keep the pot inside an unheated propagator to maintain the humidity. Keeping the soil humid helps keep the soil warm and prevents water from evaporating so the roots are more likely to grow.

Keep the pot in an area without the risk of frost. If you live in an area with freezing winters, bring your penstemon cuttings inside and let them grow over winter. Make sure they’re near a south-facing window so they get the most sun throughout the day. Otherwise, you can also keep them in a heated greenhouse so they don’t freeze.

Trim any flowers that form on your cuttings to promote better growth. Check your cuttings every few days to see if they’re developing flowers. If they are, pinch the flowers off at the base and discard them. If you can’t easily pinch them off, use your pruning shears or a gardening knife to remove them. Flowers use up nutrients that your penstemons can use to grow stronger roots or taller stems.