Repotting guide

When & how to repot Sea Knotgrass (Polygonum maritimum)

Also called Sea Knotgrass, Coast Knotgrass, Sea Knotweed.

More about sea knotgrass

About Sea Knotgrass

Polygonum maritimum · also called Sea Knotgrass, Coast Knotgrass · flowering

Polygonum maritimum is a woody-based perennial herb in the Polygonaceae family, native to the sandy and shingle beaches of the Mediterranean, Atlantic Europe, and Black Sea coasts, with rare populations on a few south-west English beaches. It forms sprawling, grey-green mats of small elliptic leaves with distinctive silvery ochrea (papery sheaths at each leaf node) and produces tiny pinkish-white flowers from June to October. Its key requirement is non-compacted, freely draining coastal sand or fine shingle; it is a specialist of very open, disturbed beach environments. This species is not listed by the ASPCA and is classified mildly-toxic as a precaution.

Mature size: 10–50 cm tall, spreading 30–80 cm wide.

Watch for — Root rot in moisture-retentive soil: The most significant problem in cultivation; plant exclusively in coarse grit or beach sand, and ensure containers have large drainage holes to prevent water accumulation at the root zone.

How to tell sea knotgrass needs repotting

Repotting on a calendar is less reliable than reading the plant. For sea knotgrass, watch for these signs:

Thick roots out of the drainage holes, or circling the surface and lifting the plant.
The pot dries out unusually fast and sea knotgrass wilts between waterings it used to shrug off.
The plant is visibly top-heavy and tips over easily.
Stalled growth and small new leaves over a full season — though with a big specimen, top-dressing is often the better first response before a full repot.

For the underlying biology of a pot-bound root system and why it stalls a plant, see our guide to spotting and fixing a root-bound plant.

How often to repot sea knotgrass

Every 2–3 years; top-dress in the in-between years. Sea Knotgrass's growth habit — prostrate to semi-erect, much-branched woody-based perennial herb, 10–50 cm tall, with stiff, silvery-grey branches and distinctive papery ochrea at each node. — sets the pace. Polygonum maritimum is a woody-based perennial herb in the Polygonaceae family, native to the sandy and shingle beaches of the Mediterranean, Atlantic Europe, and Black Sea coasts, with rare populations on a few south-west English beaches. It forms sprawling, grey-green mats of small elliptic leaves with distinctive silvery ochrea (papery sheaths at each leaf node) and produces tiny pinkish-white flowers from June to October. Its key requirement is non-compacted, freely draining coastal sand or fine shingle; it is a specialist of very open, disturbed beach environments. This species is not listed by the ASPCA and is classified mildly-toxic as a precaution.

What size pot to step sea knotgrass up to

Move up exactly one pot size. A heavy sea knotgrass dropped into a vastly bigger pot sits in a reservoir of wet soil its roots cannot reach, which rots them and destabilises the plant. In the years between repots, lift off and replace the top 3–5 cm of soil (top-dressing) instead — it refreshes nutrients without the shock of a full repot.

Not sure of the exact diameter? Our pot size calculator takes the current pot and root spread and tells you the right next size — it deliberately recommends a single step up, never a big jump.

The best time of year to repot sea knotgrass

Early spring, just as new growth restarts, is the ideal window for sea knotgrass. The plant is moving into its strongest growth phase and re-roots into fresh soil quickly. Avoid repotting in winter dormancy or, for flowering plants, while it is in bud or bloom — recovery is slowest then and you risk dropping the flowers.

Step-by-step: repotting sea knotgrass

Consider top-dressing first. If sea knotgrass is not badly root-bound, scrape off and replace the top 3–5 cm of soil instead — far less shock for a big plant that hates moving.
Get help and one size up. For a full repot, choose a pot just one size larger. A heavy plant needs two people and a stable, free-draining pot.
Ease it out on its side. Lay the plant down, slide the pot off, and gently loosen the outer roots. Do not bare-root a mature specimen.
Repot at the same depth. Add fresh coarse, well-drained beach sand or fine shingle, low in nutrients beneath and around the rootball, keeping the original soil line. Firm it so the trunk is stable and upright.
Water and leave it put. Water thoroughly, then leave sea knotgrass in the same spot and light — moving and repotting at once is what makes it drop leaves.

Aftercare

Leave sea knotgrass in exactly the same spot and light it was in before — moving and repotting at the same time is what makes a big specimen drop leaves. Water it in well, then let the top of the soil dry before watering again so the larger volume of fresh soil does not stay sodden. Do not fertilise for about 4 weeks — fresh mix already carries nutrients and feeding freshly disturbed roots scorches them.

The right soil mix for sea knotgrass

Sea Knotgrass wants coarse, well-drained beach sand or fine shingle, low in nutrients. Authentic beach sand mixed with pea gravel is the ideal growing medium; standard potting compost is too moisture-retentive and nutrient-rich, leading to root rot and rank, uncharacteristic growth. Always use fresh mix when you repot — reusing old, broken-down soil reintroduces the compaction and poor drainage you are repotting to fix.

Repotting sea knotgrass — frequently asked questions

How often should you repot sea knotgrass?

Every 2–3 years; top-dress in the in-between years for sea knotgrass. Fully repot sea knotgrass only every 2–3 years; in the in-between years just top-dress the top 3–5 cm of soil. Step up one pot size in spring with coarse, well-drained beach sand or fine shingle, low in nutrients. It is heavy and hates being moved, and a vastly oversized pot holds water against the roots and rots them.

What size pot does sea knotgrass need?

When is the best time of year to repot sea knotgrass?

Should you top-dress or fully repot sea knotgrass?

For a big, heavy sea knotgrass, top-dressing — replacing the top 3–5 cm of soil — is the gentler option most years, with a full repot only every 2–3 years. A mature specimen sulks and drops leaves when fully repotted, so do it as rarely as the roots allow.

Should you fertilise sea knotgrass after repotting?

Not immediately. Wait about 4 weeks after repotting sea knotgrass. Fresh mix already contains nutrients, and feeding freshly cut or disturbed roots burns them. Resume your normal feeding routine once you see new growth.