Yellow flag (Iris pseudacorus) is a flowering perennial plant native to Europe, Asia, and Africa. In temperate climates throughout the globe, yellow flag is cultivated as an ornamental plant. Yellow flag contains the toxin tannin and can irritate the skin. It’s considered and invasive aquatic species.
I. Appearance and Characteristics
Iris pseudacorus, the yellow flag, yellow iris, or water flag, is a species of flowering plant in the family Iridaceae. It is native to Europe, western Asia and northwest Africa. Its specific epithet pseudacorus means “false acorus”, referring to the similarity of its leaves to those of Acorus calamus (sweet flag), as they have a prominently veined mid-rib and sword-like shape. However, the two plants are not closely related. The flower is commonly attributed with the fleur-de-lis.
This herbaceous flowering perennial plant grows to 100–150 cm (39–59 in), or a rare 2 m (6 ft 7 in) tall, with erect leaves up to 90 cm (35 in) long and 3 cm (1.2 in) broad. The flowers are bright yellow, 7–10 cm (2.8–3.9 in) across, with the typical iris form. The fruit is a dry capsule 4–7 cm (1.6–2.8 in) long, containing numerous pale brown seeds.
Iris pseudacorus grows best in very wet conditions, and is common in wetlands, where it tolerates submersion, low pH, and anoxic soils. The plant spreads quickly, by both rhizome and water-dispersed seed. It fills a similar niche to that of Typha and often grows with it, though usually in shallower water. While it is primarily an aquatic or marginal plant, the rhizomes can survive prolonged dry conditions.
Large I. pseudacorus stands in western Scotland form a very important feeding and breeding habitat for the endangered corncrake.
Iris pseudacorus is one of two iris species native to the United Kingdom, the other being Iris foetidissima (stinking iris).
Both the petals and stem are toxic to animals and plants.
II. How to Grow and Care
Sunlight
Yellow flag likes sufficient sunlight. It needs to receive more than 6 hours of light per day and can be planted in a garden with no shade. It can also grow in slightly shady environments, but the quality of its blooms and disease resistance may decrease. Some varieties tolerate environments with less light and can grow in the shade of other plants.
Temperature and Humidity
Yellow flag iris tolerates temperatures to 0°F, and it grows best in areas with high humidity. Gardeners in arid regions can mist their plants regularly to increase humidity.
Watering
This semi-aquatic plant likes wet soil—it is difficult to overwater it. Yellow flag iris can survive in soil that is only moderately moist, but it produces fewer flowers when you grow it in less-than-wet conditions. Don’t let the soil dry out completely.
Soil
Yellow flag likes loose, fertile, and well-drained soil, but can also grow in dry and barren earth. It likes slightly acidic or neutral soil with a pH value of 6.0-7.0. Sulfur can be added to improve alkaline soil, while lime can be added if the soil is acidic.
Fertilizing
Yellow flag likes soil rich in humus, so fertilize in the early spring, early fall, and before its flowering period. Before it blooms, use phosphorus-rich compound fertilizers, such as fertilizers with a 5-10-5 NPK ratio, to promote blooming and encourage more gorgeous flowers. Balanced fertilizers (20-20-20 NPK ratio) can also be used. Some organic fertilizers contain fewer nutrients but improve the soil, so it is also good for the plant’s growth. Water right after fertilizing to help the roots absorb the fertilizer.
Planting Instructions
It’s best to plant yellow flag in the summer. Choose a spot with ample sunlight and good drainage, and turn up the soil to 25 cm deep. Then topdress with fertilizer and mix in evenly. When planting, you can cut the leaves to 15 cm in length, which helps reduce water evaporation and increase the survival rates. Finally, cover the rhizome in the soil. Avoid letting the roots get dehydrated for too long during planting, and water right after planting.
Pruning
During the growing season, remove bloom-bearing stalks after the flower dies. In the fall, prune any old or dead leaves, cutting the entire plant back to 1 inch for winter.
Propagation
Yellow flag spreads by fast-growing underground rhizomes that make it easy to divide.
- After the blooming season ends, use a spade to dig up a clump of yellow flag irises.
- Brush off some of the soil and use your hands or a sharp knife to separate the rhizomes into sections containing rhizomes and healthy leaves.
- Trim off the top half of the leaves.
- Dig a shallow hole in moist, rich soil, and bury the rhizomes only halfway; then press them into the soil. Don’t plant them deep.
- Water well and keep the soil moist.
How to Grow from Seed
Yellow flag can also be grown from seed.
- Harvest the orange-brown seeds when the seed pods ripen and open in the fall.
- Soak them in warm water for 24 hours before planting.
- Sow the seeds on moist, rich garden soil immediately for the best chance of germination the following spring.
- Cover the seeds lightly with about 1/8 inch of soil.
- After the seeds germinate, wait several weeks for the seedlings to grow large enough to be transplanted to their permanent garden site.
- It can take anywhere from 1 month to 6 months for the seeds to germinate.
Overwintering
After the foliage dies back completely in fall, trim the leaves to 1 inch. Don’t apply a layer of mulch over yellow flag iris plants as winter approaches. If the winter is particularly frigid, spread straw around the base of the plant.
How to Get to Bloom
Little work is required to encourage yellow flag iris to bloom. As long as it has optimum growing conditions, gardeners can expect blooms ranging from rich dark yellow to pale yellow in late spring to early summer. The 3–6 inch blooms are not fragrant, but they are showy in the garden and last for a couple of weeks. After a flower dies, deadhead it, an action that sometimes prompts a second bloom.
III. Uses and Benefits
- Ornamental uses
Yellow flag is highly popular as a wetland perennial in warmer climates. It is very easy to grow and produces a lot of flowers. However, it should not be planted near waterways that feed into wild areas because it can spread and become invasive. Gardeners should stick to using it in wet soils, or by enclosed ponds. Plantain lilies, Bistort, and Solomon’s seal provide a good backdrop for this pretty flower.
- Other uses
This plant has been used as a form of water treatment since it can take up macronutrients (such as nitrogen and phosphorus) through its roots, and is featured in many AS Level Biology practicals as its ability to grow in low pH levels makes it a useful indicator.
It can also withstand high salinity levels in the water.
IV. Harvesting and Storage
Yellow flag flowers can be harvested for bouquets. Choose not-fully-bloomed flowers and cut them off from the stem base obliquely with a pair of sharp gardening scissors. Quickly put the flower in a vase with clean water to avoid its dehydration.
