Mertensia maritima Silver Ocean with silver-blue leaves and nodding sky-blue flowers in a gritty coastal-style garden

How to Grow Mertensia Silver Ocean from Seed in South Africa

Want a fresh, unusual herb for a cool edible pot or a silver-blue coastal-style bed? Learn how to grow Mertensia maritima ‘Silver Ocean’ from raw seed, with the cool germination, gritty soil, careful watering and winter protection this slow-growing perennial needs in South Africa.

Mertensia maritima ‘Silver Ocean’

‘Silver Ocean’ is a low, spreading perennial oyster leaf with fleshy silver-blue-green leaves and nodding blue, bell-shaped flowers. It is happiest where roots drain quickly but the plant does not dry out completely. The three current options on the linked product are 10, 100 and 1000 raw seeds; raw describes the physical seed form and does not by itself confirm chemical-treatment status.

Quick Grow Facts

Botanical name: Mertensia maritima, family Boraginaceae
Growth form: Low, trailing/spreading, deciduous perennial; allow about 20 cm height and 40–50 cm spread
Seed form: Raw seed; no soaking is required
Position: Bright light or morning sun with protection from harsh midday heat; a little afternoon shade is useful inland
Medium: Fine seedling mix with added perlite or grit, pH about 5.5–6.3; sharply drained after emergence
Sowing depth: Press seed onto the surface and cover only lightly with fine vermiculite, about 1–2 mm
Germination: Keep the medium evenly and consistently moist at 20–22°C; allow roughly 10–20 days, with uneven emergence possible
Transplanting: Move a root-held plug at 5–6 weeks; harden off over 7–10 days and disturb the root ball as little as possible
Spacing: 40 cm between plants, or 50 cm where wet-season airflow is a concern; use one plant per small pot
Flowering: Usually needs a cool winter period to initiate flowering; allow about 10–15 weeks to finish after establishment, then longer if grown for a mature clump
Use: Edible leaves and flowers when grown in clean, food-safe conditions; also attractive in rock gardens and containers

How to Sow

Fill a clean tray or small pot with moist, fine, free-draining mix. Do not use garden soil for germination.
Place the raw seeds on the surface and press them in. Add a very thin vermiculite veil rather than burying them deeply.
Keep at 20–22°C in bright, indirect light. Cover loosely with a clear lid or plastic with ventilation holes, removing it as soon as seedlings appear.
Keep moisture steady but avoid stagnant, waterlogged conditions. Reduce moisture slightly after emergence and provide gentle airflow.

Do not discard the tray after one week: emergence can be irregular. If there is no movement after 20 days, check that the seed was not buried, the mix did not dry out, and nights were not too warm or too cold. A cool period is more important for later flowering than for the first emergence of fresh commercial seed, so do not chill seed unless your packet or supplier gives a different instruction.

Transplanting and Spacing

Transplant when the plug holds together and the young plant has several true leaves, usually at 5–6 weeks. Harden it gradually for 7–10 days, beginning in bright shade and increasing morning sun. Set the crown at the same level as in the tray. Give 40 cm spacing for a compact planting; choose 50 cm in humid areas or where winter rain is heavy. A 20–25 cm pot suits one plant, provided it has generous drainage holes.

Light, Water and Soil

Use gritty, low-fertility soil or a mineral-rich container mix. Full sun can work in cool coastal conditions, but morning sun and afternoon shade are safer in hot interior gardens. Keep plants moderately moist while they root. Once established, water deeply when the upper layer begins to dry, then let excess water drain fully. Yellow leaves usually mean the plant has been kept too dry; soft, collapsing leaves and crown rot point to poor drainage or wet foliage overnight.

Feed lightly with a balanced, low-strength fertiliser during active growth. Avoid heavy nitrogen, which produces soft foliage. Good airflow is part of disease prevention: space plants, water the soil rather than the leaves and remove damaged foliage promptly.

South African Regional Timing

Winter-rainfall Western Cape: Sow in late summer to early autumn while days are mild, then protect seedlings from prolonged winter saturation. Raised beds and pots are often safer than heavy ground.
Frost-prone interior: Sow in late winter or early spring under cover, transplant after severe frost risk has passed, and give afternoon shade as spring heat rises.
Humid coast and KZN Midlands: Use extra grit, open-sided shelter and generous spacing. Avoid wet foliage after sunset and protect the crown from persistent rain.
Hot-dry and subtropical interior: Sow during the coolest part of the year, use morning sun or filtered light, and keep the root zone evenly moist without allowing the pot to stand in water.

Flowering, Harvest and Maintenance

Expect a low clump of silver-blue leaves and blue flowers after the plant has experienced a cool season. Harvest a few young leaves once the plant is well established; take no more than one-third of the foliage at a time. The leaves and flowers are commonly used raw as a garnish or in salads, with a salty, oyster-like flavour. Stop harvesting heavily when flower stems are forming if you want the best display. Cut back spent stems after flowering and keep the plant tidy, but avoid hard pruning into the crown.

Troubleshooting

No seedlings after 10–20 days: Likely causes are deep sowing, a dry surface, stale or poorly stored seed, or a temperature outside the 20–22°C range. Reseed shallowly in a fresh, fine mix and keep the surface evenly moist.
Seedlings collapse at the base: Usually excess moisture, poor airflow or contaminated mix. Remove the cover, improve ventilation, water earlier in the day and use clean, free-draining media.
Leaves turn yellow: Plants can yellow when kept too dry, but saturated roots can show similar pallor. Check moisture below the surface, then adjust irrigation and drainage rather than adding fertiliser immediately.
Leaves soften or rot in winter: Move containers under rain cover, raise beds, improve grit and keep water away from the crown. Winter wet is more dangerous than cool temperatures.
Flowering is delayed: The crop may not yet have received a sufficiently cool season. Keep it bright and moderately moist through winter, and avoid forcing lush growth with high nitrogen.
Slugs and snails chew leaves: Inspect at dusk, remove hiding places and use only a locally registered control exactly as its label permits. Do not apply any product to leaves intended for eating unless its label allows that use.

Safety

Although oyster leaf is traditionally eaten, only harvest plants grown in clean, food-safe media and with products approved for edible crops. Never eat the raw seed, and do not assume “raw seed” means untreated. Keep seed and garden chemicals away from children and pets; if you are pregnant, have allergies or are unsure about identification, ask a qualified professional before eating any plant material.

Brought to you by Ubuntuberg Seeds, where our mission is to cultivate humanity & harvest unity. Ubuntuberg Seeds: the highest quality seed stock in all of Southern Africa, sourced from sustainable farms, guaranteeing you the best quality seed stock.