Persian shield (Strobilanthes dyerianus) is grown for its large leaves. The leaf can grow 4-7 inches long and ends at a point. They are slightly serrated. Each purple to silver leaf has dark green veins and is iridescent. In the fall to winter, the Persian shield sometimes grows purple-blue flowers. Persian shield got its name because each leaf looks like the battle shield Persians used with its purple iridescent color and a silvery metallic sheen. Persian shield can grow up to 4 feet tall and 3 feet wide with a bushy habit. While it is an evergreen perennial in the tropics, it is commonly grown as an annual or an indoor plant in colder areas.
Persian shield grows best in part shade to shade. It requires rich, moist soil. When grown as an indoor plant, it needs bright light, or the leaf colors will fade. Houses are generally too dry for Persian shield plants, so set the flowerpot on a tray of pebbles. Fill the tray with enough water to rise halfway up the pebbles, but not enough to touch the pot. As the water evaporates, it creates enough humidity to keep the plant happy. Don’t forget to refill the tray when the water gets low.
The Persian shield is an easy plant to grow. The sun glinting off the leaves makes quite a show outside.
About Persian Shield
Persian shield is native to Myanmar (Burma). This plant is a member of the acanthus family, a large family of sub-tropical plants with about 2,500 species, mostly in tropical or sub-tropical areas. All the plants in the family have two-ranked leaves and a unique flower arrangement that is four-sided. The flower grows inside bracts at the end of the stalk in a little cluster.
Persian shield plants flower inconsistently. The things that control flowering in most plants, such as length of light and dark, temperature, or amount of food, do not seem important to Persian shield. Even in controlled conditions, the mechanism controlling flowering has not been discovered. In addition, one problem is that after the flower stalks begin to grow, the plant stops growing leaves, the stems become woody, and flowers arise from every node. This ruins the plant for propagation purposes. Most people pinch off the flower stalks before the flower blooms to prevent this problem.
How to Grow the Persian Shield
Because of the inconsistency of flower production, seeds are hard to come by. Persian shield is almost always purchased as a plant from a nursery. Plant it outdoors when the soil has had a chance to warm up and the nights are no longer cold. Persian shield looks very nice beside silver or gray plants that set off its purple. It can be the focal specimen in a container or as part of a mass planting. It is often used as a border or edging plant.
How to Propagate the Persian Shield
Persian shield is propagated through stem cuttings. Cut a 2–3-inch section of the stem, cutting just below the growth node. Strip all but the top two leaves from the stem. Plant the stem in a soilless medium such as peat moss. Mist the plant and place a clear plastic bag over it. Place in a warm spot. A seed germination mat works well to help the plant grow roots without rotting first. Remove the plastic bag for an hour each day to guard against mold. When the cutting has roots, plant it in a regular potting medium.
You can also root the cutting in water placed on a warm windowsill. Change the water every two days. The cutting will be ready to pot when the roots appear long enough to support the rest of the plant, usually a week to ten days.
Caring for Your Persian Shield
Keep Persian shield moist but not soggy. The roots will rot if the soil is soggy. If you grasp a handful of soil and squeeze, only a drop or two should come out. You can also tell with a moisture meter when the plant needs watering. It quickly wilts when too dry and may not recover even after watering. Inside, water from the bottom by setting the pot in an inch of water for 20 minutes.
In-ground plants should be fertilized at the start of the growing season. Use a balanced fertilizer such as a 10-10-10. Another dose of fertilizer is needed halfway through the growing season. If you are growing the plant in a pot, use a water-soluble fertilizer mixed at half strength every two weeks.
Outside, plant Persian shield in part shade to shade. In part shade, the sunlight will glint off the leaves and they will really shine. The shade should be in the afternoon in hot climates, so it does not scorch the leaves. Inside, the plant needs to be in bright light, or the colors of the leaves will fade. Use caution when placing the plant in a windowsill to make sure it does not get too hot or cold.
Persian shield needs rich soil. If the soil is not rich, till compost into the soil to improve it. The compost will improve not only the nutrients available but aerate the soil and improve water retention. Compost also breaks up the soil so the roots can grow in the spaces that open up. The soil also needs to be well-drained so that water does not pool around the plant and the plant’s roots. This will cause root rot and kill the plant. In a pot, it is best to use a soilless potting medium to grow Persian shield in. The medium is light and allows the roots to easily move through the potting medium while they are growing.
Persian shield is a sub-tropical plant. It grows well in hot and humid areas. It is hardy in USDA zones 8-11. It may die back to the roots in cold weather, but when it warms up in the spring the plant will put out new leaves. Everywhere else, Persian shield will need to be treated as an annual and be replanted each spring. The outstanding foliage on this plant makes that an attractive option. You can also grow Persian shield as a house plant. As long as the house is comfortable for you, the house will be comfortable for the plant. You will have to provide more humidity for the plant, as mentioned above.
Pruning Your Persian Shield
The Persian shield plant does not need pruning. However, if you pinch back the stems early in the growing season, your plant will be bushier and look better. It can get pretty leggy if you do not do this. If you are growing Persian shield as an annual and you pinch back the stem tips, you won’t have this problem. Indoors, once the plant is mature, you can prune the roots during repotting so that it doesn’t become root-bound. It also helps control the size of the plant. If the mother plant becomes too leggy in your pot, start several cuttings. When they are ready, throw the leggy mother plant out and grow the best of the stem cuttings.
The best way to control the size a Persian shield house plant grows to is the size of its pot. Young plants should be re-potted every year with a larger pot. When the plant is as large as you want it, stop increasing the size of the pot. You will still need to re-pot it every other year, but you can sterilize the current pot with bleach water and rinse it well, then re-use it with fresh potting medium. Always use fresh potting medium when re-potting any plant, including the Persian shield. You will need to trim the roots of the mature plant before re-potting it so it doesn’t get so root bound the roots grow in a circle and girdle the plant.
Persian shield can get root rot from too much water. Because aphids and mealybugs like Persian shield, you may see some sooty mold on the leaves. It is black and sooty looking and will eventually totally cover the leaf. In addition to looking bad, the mold blocks the sunlight from the chlorophyll, so the plant slowly starves to death. First, get rid of the aphids and mealybugs. Then wash the leaves off with a soft cloth dipped in water with a few drops of Dawn dishwashing soap in it.
For the most part, Persian shield is not troubled by pests. Deer and rabbits do not bother this plant under normal circumstances. Indoors, Persian shield sometimes gets the following pests, which will suck the sap out of the leaves. Leaves will eventually turn yellow and drop off the plant if not treated.
Mealybugs are small, white bugs that often look like small cotton fuzz sticking to the stem or leaf. They are related to scale insects. Mealybugs excrete honeydew, which both ants and sooty mold love. Treat the plant in Neem oil to smother the mealy bugs. Then wash the sooty mold off the leaves.
Aphids are small bugs that also excrete honeydew. Aphids are usually green but also come in pink, white, grey, or black. Ants will actually bring aphids to plants so they can eat the honeydew the aphids excrete. Sooty mold likes to grow where there are aphids, too. Neem oil will suffocate aphids. Once they are gone, you can wash off the sooty mold.
Ants like honeydew and will protect the aphids and mealybugs from predators so they can eat it. If you have an ant problem in your potted plants, treat the aphids and mealybugs with Neem oil. When the aphids and mealybugs are gone, the ants will no longer bother the plant. You may have to repot the plant after washing away the ants with plain water. Wear gloves and be cautious so you do not get stung by the ants.
Fungus gnats are small, dark flying insects. Adults do not bite, sting, or spread diseases to humans, but can be incredibly annoying. Fungus gnat larvae eat the roots of plants. They can kill the plant or stunt its growth when present in large numbers. Fungus gnats also spread like wildfire into every potted plant in the house. They require moist soil to lay their eggs in. If you water your plant from the bottom, the top ½ inch of soil will dry out and kill the fungus gnats. You can also water the soil with a cup of water with several drops of dishwashing soap in it. The soap will kill the larvae.
Spider mites are small red mites that produce very small webs, usually where a leaf stalk joins the rest of the plant. Spider mites like dry, dusty leaves. Make sure you periodically wipe the leaves with a damp cloth. It is important to keep the plant well hydrated, so the leaves are not dry. Because these are mites, not insects, regular insecticides do not work on them. Spraying the leaves with Neem oil will kill the spider mites. Make sure you get the underside of the leaf and the area where leaf stems join the rest of the plant. It may take several treatments to get rid of the mites, so be patient.
Safety For Pets
There is some question as to whether Persian shield is poisonous. Some sites say it is poisonous to dogs, cats, and livestock. The tubers and roots are listed as especially toxic. However, Persian shield is not on the ASPCA’s list of poisonous plants. You should not allow your pets to eat this plant, just in case.
Persian shield is a low maintenance, easy to grow plant. The foliage is striking, and it looks good as a mass planting, mixed in a container or shade garden with grey or silver plants, or as a houseplant. This plant is easy to propagate from a cutting, but it is hard to find seeds to grow. It has few pests and those it has are easily controlled.