Yellow flag

Yellow flag: how do you recognise it?

Habit

Upright clumps up to about 1 m tall and 60 cm wide, which spread quickly.

Leaves

Sword-shaped, greyish green to fresh green, stiff and 1.5–3 cm wide.

Flower

Bright yellow, 7–10 cm across, with brown or purplish-brown veins on the drooping falls; several flowers per stem.

Fruit

Green, three-sided capsules holding flat brown seeds that float on water.

Roots

A thick, branching rhizome just below the surface; poisonous.

Flowering: May–June · Plants: March–May and September–October · Pruning: July and November

Why grow it?

• Bright yellow flowers in May–June: one of the most striking sights at the pond edge.
• Native and good for bees and bumblebees.
• The sturdy, upright leaves give structure to the bank, even after flowering.
• Grows in wet soil as well as in shallow water: a marginal plant for any pond.
• Takes up a lot of nutrients from the water, which helps to keep algae in check.
• Tough, hardy and easy.

Yellow flag: where does it grow best?

• Sun to partial shade; it flowers best in full sun.
• Wet, fertile soil: loam, clay or peat; pH does not matter.
• On the bank, in boggy ground or up to 20 cm under water.
• Allow 50–60 cm per plant; in a small pond, grow it in a basket to keep it in check.

Light

Sun and partial shade

Soil type

Loam, clay or peat

Acidity

Not fussy

Moisture

Wet

Size

1 m tall, 0.6 m wide

Hardiness

Hardy (to -30 °C)

Goes well with

• Marsh marigold: Flowers in April, before the yellow flag, in the same wet spot.
• Purple loosestrife: Purple spikes in July–August, once the iris has finished.
• Water forget-me-not: Small sky-blue flowers at its feet, in the same shallow water.
• White water lily: Calm on the open water behind the tall iris leaves.
• Flowering rush: Pink flower heads in summer, also in shallow water.

Yellow flag: when and how to plant it?

Plant in March–May and September–October.

1. Plant from March to May or in September–October.
2. On the bank: dig a hole in wet soil and set the rhizome just below the surface.
3. In the pond: plant in an aquatic basket with heavy soil, top with gravel and place the basket so there is 0–20 cm of water above the soil.
4. Space plants 50–60 cm apart.
5. Water bankside plants well straight away and keep the soil wet until they are established.
6. Wear gloves: the sap of the rhizome can irritate the skin.

Care

Watering

On the bank, the soil must stay wet at all times; in dry summers, water or top up the pond.

Feeding

Not needed: it gets enough nutrients from the soil and the water.

Faded flowers

Cut off the flower stems after flowering if you do not want seedlings.

Dividing

Every 2–3 years in spring, or the clumps become too large.

Old leaves

In November, cut the dead foliage back to 10–15 cm, above the waterline.

Tip: In a small pond, always plant it in a basket: that keeps it within bounds and makes it easy to lift out for dividing.

Yellow flag: when to prune it?

Pruning months: July and November.

When: In July, after flowering, cut off the flower stems. In November, remove the dead foliage.

How:

• Cut faded flower stems off just above the leaves, before the seed pods ripen.
• You can leave a few seed pods for winter interest, but it will then self-seed.
• In November, cut the brown leaves back to 10–15 cm, above the waterline.
• Take dead foliage out of the water so it does not rot.

Caution: Wear gloves: the sap can irritate the skin.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Division

In March–April, dig up the clump or lift the basket and split the rhizome with a knife or spade into pieces, each with a few fans of leaves. Trim the leaves to 20 cm and replant straight away.

Seed

In autumn, sow fresh seed in a pot of wet compost and leave it outside: winter cold helps germination. Seedlings take a few years to flower.

Yellow flag: how does it get through the winter?

Hardy to −30 °C.

• Fully hardy to about −30 °C: no protection needed.
• The leaves die back in autumn; the rhizome overwinters in the wet soil or under water.
• Plants in baskets in shallow water can stay where they are, even if the pond freezes over.

Diseases and pests

• Iris sawfly: Greyish-green larvae eat the leaves from the edges in May–June. Pick them off wearing gloves or knock them into the water for the fish to eat.
• Leaf spot: Brown spots on the leaves, especially in wet summers. Cut off affected leaves and clear away old foliage.
• Too vigorous: In a small pond it soon takes over the whole margin. Grow it in a basket, divide regularly and remove the seed pods.
• Few flowers: Too much shade or an overcrowded clump. Divide the plant and move it to a sunnier spot.

Through the year

• March: Plant or divide.
• April: Plant or divide. The new leaves are growing.
• May: Flowering. Watch out for iris sawfly larvae.
• June: Flowering. Bees and bumblebees visit the flowers.
• July: Cut off faded flower stems before they set seed.
• September: Plant. Sow fresh seed.
• October: Plant. The leaves start to change colour.
• November: Cut dead foliage back to 10–15 cm, above the waterline.

Edible and useful

Not edible. All parts of yellow flag are poisonous, especially the rhizome: they cause vomiting, stomach ache and diarrhoea, in dogs, cats and livestock too. When not in flower its leaves look very like those of sweet flag (also poisonous): yellow flag has no scent when you crush a leaf, sweet flag smells spicy. Never eat anything from a waterside plant you are not sure of. The sap can irritate the skin: wear gloves when dividing or cutting it back, and do not let children chew the rhizome. If any is swallowed, call a poison centre (in Belgium 070 245 245) or your doctor at once.

Good for wildlife

Bees

Nectar and pollen for honeybees and wild bees.

Did you know?

• Yellow flag is often regarded as the model for the fleur-de-lis.
• The seeds float thanks to an air layer under the seed coat: that is how it spreads along streams and rivers.
• Outside Europe, for instance in North America and New Zealand, yellow flag is an invasive alien; here it is native.