Spiked water-milfoil

Spiked water-milfoil: how do you recognise it?

Habit

Long, branching stems that lengthen with the water depth and spread out just below the surface; one plant soon covers 1 m across.

Leaves

In whorls of usually four around the stem, feathery and divided into thread-like segments; olive to dark green, sometimes reddish.

Flowers

Small, inconspicuous reddish flowers on an upright spike that rises just above the water; they are wind-pollinated.

Roots

Fine roots at the stem base and at the nodes; broken-off pieces root again easily.

Pond zone

Submerged plant or oxygenator for 30–150 cm depth.

Plants: April–August · Pruning: July–August

Why grow it?

• A strong oxygenator: the fine leaves take up nutrients from the water, which holds back algae.
• Also grows in deeper water, down to 1.5 m, where few other plants manage.
• Fish lay their eggs among the fine leaves, and young fish, tadpoles and water insects shelter there.
• Native and easy: a bunch of stems weighted to the bottom with a stone is all it takes.
• Its fine, feathery foliage is surprisingly beautiful underwater in a clear pond.

Spiked water-milfoil: where does it grow best?

• Sun or partial shade; in deep shade it grows slowly.
• Clear, neutral to lime-rich water; it does less well in soft, acid water.
• Planting depth 30–150 cm below the surface, on the bottom or in a basket.
• Loam or clay on the pond floor, or a basket of heavy soil; no potting compost.
• A few bunches are enough for an ordinary garden pond: it fills out quickly.

Light

Sun and partial shade

Soil type

Loam, clay or peat

Acidity

Neutral or chalky

Moisture

Wet

Size

0.05 m tall, 1 m wide

Hardiness

Hardy (to -25 °C)

Goes well with

• Yellow water lily: Large floating leaves above the fine foliage, in the same deep zone.
• Frogbit: Floats on the water and casts shade, holding back algae even more.
• Mare’s-tail: Straight stems in shallower water, around the oxygenators.
• Fringed water lily: Yellow flowers on the water in summer, in the same lime-rich water.

Spiked water-milfoil: when and how to plant it?

Plant in April–August.

1. Plant from April to August, once the water has warmed and growth is rapid.
2. Tie a bunch of five to ten stems, 15–20 cm long, at the base to a small stone with a piece of string; preferably avoid lead weights.
3. Lower the bunch to 30–150 cm deep so the stone holds it on the bottom, or push the base 5 cm deep into a basket of heavy soil.
4. Top a basket with gravel so fish do not dig up the soil.
5. Rinse new plants first and check them for unwanted plants, snail eggs and blanketweed.
6. A few bunches are enough; it fills out a great deal in one season.

Care

Thinning

In July–August, rake or pull out some of the stems if it fills more than a third of the pond.

Feeding

Never: it takes its nutrients from the water and the pond floor.

Algae

Twist blanketweed out from between the stems with a stick.

Water level

Top up with rainwater so the water stays clear and low in nutrients.

Autumn

Net fallen leaves out of the pond so no thick layer of sludge builds up.

Tip: Thin it out every summer so it fills no more than a third of the pond: that way it stays a useful oxygenator rather than a thug.

Spiked water-milfoil: when to prune it?

Pruning months: July–August.

When: Thin out in July and August, when it takes up too much space.

How:

• Pull or rake out part of the stems; always leave a good amount as an oxygenator.
• Cut or pull away stems that form a dense mat at the surface.
• Leave thinned stems on the bank for a day so small creatures can crawl back into the pond.
• Gather broken fragments carefully: every piece can root again.

Caution: Put thinned stems on the compost heap, never into a ditch, stream or pond in the wild.

Pruning: when and how · Pruning calendar and basic rules

Propagation

Stem cuttings

From April to August, tie 15–20 cm pieces of stem to a small stone and let them sink, or push them into a basket. They root within a few weeks.

Division

In spring, lift a rooted clump from a basket, split it and replant the pieces.

Spiked water-milfoil: how does it get through the winter?

Hardy to −25 °C.

• Hardy to about −25 °C: it overwinters underwater on the bottom.
• In autumn some of the stems die back; in spring new shoots grow from the base.
• Make sure the pond does not freeze to the bottom; in a garden pond with a deep section that is rarely a problem in a Belgian winter.

Diseases and pests

• Spreading: In a small, nutrient-rich pond it soon fills all the water. Thin out every summer.
• Blanketweed: Forms woolly masses between the stems. Twist it out with a stick and tackle the cause: no fertiliser and not too much fish food.
• Fish: Goldfish and koi eat the young shoots and pull the bunches loose. Plant in a basket and protect young bunches with mesh if needed.
• Limp, bare stems: Too little light or water that is too soft. Move it to a sunnier spot or shallower water.

Through the year

• April: Plant: sink bunches weighted with a stone.
• May: Plant. Rapid growth begins.
• June: Plant. Remove blanketweed.
• July: Thin out if it fills more than a third of the pond.
• August: Thin out. Last month for planting.
• October: Net fallen leaves out of the pond.
• December: Resting on the pond floor.

Did you know?

• In North America, where it was introduced, spiked water-milfoil has become one of the most troublesome invasive aquatic plants.
• Myriophyllum means ‘countless leaves’ in Greek, after the finely divided foliage.
• The flowers are wind-pollinated, which is why the flower spike rises above the water.