When to plant rosemary in RHS H4 (UK)

Key dates for rosemary at RHS H4

Dates are typical for the regions H4 describes (Most of southern England, the south Midlands, south Wales, and coastal areas across the rest of the UK). UK frost pockets, urban heat, and coastal moderation can shift the planting window by 1-2 weeks within the same rating band. Always cross-check against your local Met Office station for current conditions.

Why this timing works at H4

H4 describes UK gardens with winter minima of minimum -10 to -5 °C. Last spring frost typically passes late April; first autumn frost arrives late October, giving about 180 frost-free days.

Start seeds indoors 10–12 weeks before the last frost; germination is slow and erratic (14–21 days at 18–21 °C / 65–70 °F) with low viability, so propagation by stem cuttings is preferred by most Extension services. Transplant outdoors after the last frost once soil has warmed — rosemary is perennial only in USDA zones 7–10 (established plants survive to about −12 °C / 10 °F); in zones 6 and colder treat as a tender annual or overwinter potted plants indoors before the first autumn frost. Tip harvests of stem ends begin around 80–100 days from transplant.

UK-specific tips for H4

• H4 is the typical southern English garden — rosemary grown outdoors here perform similarly to USDA zone 8b/9a Americans seeing in their references.
• Wet July-August stretches drive blight and powdery mildew — water early in the day, mulch to reduce splash, and remove lower foliage as plants establish.
• Slug pressure peaks in May after the first warm wet spell — protect young transplants for the first three weeks with copper tape or wool pellets.
• Light intensity is lower than in equivalent USDA zones — UK summer days are long, but cloud cover means more total hours but lower instantaneous photon flux than a US zone 8 garden. Plants tend to be leafier and slower to ripen.

Quick-grow guide

• Sun: Full sun — 6+ hours direct.
• Soil temperature for germination: 18–21 °C (65–70 °F).
• Spacing: 18–36 inches (45–90 cm).
• Days to harvest from transplant: ~90 days.
• Wait until soil reaches 10-12 °C before transplanting outdoors — cold UK soils stall tender crops for weeks.

Common mistakes — H4 × rosemary

• Treating UK climate like the US zone 8: although temperature minima match, UK summers are cooler, wetter, and cloudier. Add 1-2 weeks to days-to-harvest figures from US sources.
• Sowing into cold wet soil: UK spring soil holds water longer than equivalent US zones. Wait for soil to dry enough to crumble in your hand before sowing.

Source and methodology

RHS hardiness rating thresholds from the official RHS reference. Typical frost-date averages from Met Office regional climate summaries for the geographies H4describes. Crop timing offsets calibrated against UK extension references (RHS sowing calendar, Garden Organic, James Wong's UK growing tables) and cross-checked against US Cooperative Extension Service publications. For American readers cross-referencing, RHS H4 is roughly equivalent to USDA zone 8. Curated by the Growli editorial team.

Keep going

• How to grow rosemary — full guide
• RHS H4 — typical regions and what else to plant
• All RHS hardiness ratings (H1a-H7)
• USDA hardiness zones — for cross-reference with US sources
• Fungus gnats in UK houseplants — guide

Same crop, nearby ratings

• When to plant rosemary in RHS H2
• When to plant rosemary in RHS H3
• When to plant rosemary in RHS H5
• When to plant rosemary in RHS H6

Other crops at H4

• When to plant tomatoes in H4
• When to plant peppers in H4
• When to plant basil in H4
• When to plant garlic in H4
• When to plant lettuce in H4
• When to plant bush beans in H4