Building a Cedar Greenhouse for Productive Gardening

Why Cedar is the Craftsman's Choice

Western red cedar has long been the preferred timber for greenhouses on British period properties, and for good reason. Unlike softwood, which needs regular chemical treatment to stave off rot, cedar contains natural oils that resist fungal decay and insect attack. It is also dimensionally stable, meaning it will not twist or warp as dramatically when the humidity swings from a damp January to a scorching July. For a productive garden, that stability matters: a twisted frame means cracked panes and draughts where you least want them.

Cedar also helps regulate humidity inside the greenhouse. The wood absorbs moisture from the air on damp days and releases it when the sun returns, buffering the extremes that can lead to botrytis, damping off, and other fungal woes. This is a subtle but real advantage over aluminium or plastic frames, which tend to drip condensation onto foliage. Finish the cedar with a breathable oil or leave it to weather to a soft silver-grey – either suits a period property, whether a Victorian villa or a stone cottage.

Siting and Glazing for Winter Light

The whole point of a greenhouse in the British climate is to capture low winter sun and hold onto it. Position your cedar greenhouse with its ridge running east to west, so the long south-facing side catches the maximum light from October through March. Avoid the temptation to tuck it behind a shed or under a deciduous tree; even bare branches cast shadows that can reduce light levels by a third. If you are attaching the greenhouse to a period wall, a south or south-east aspect is ideal, and the masonry will act as a thermal store, releasing warmth overnight.

For glazing, traditional 4mm horticultural glass is hard to beat. It lets through around 90% of available light, does not scratch, and lasts for decades. Twin-wall polycarbonate is lighter and safer, but it diffuses light and can yellow over time – better for a cold frame than a principal greenhouse. If you are building a lean-to against a period property, consider a solid north wall insulated with wood fibre or sheep's wool to reduce heat loss. Angle the roof glazing at 30 to 45 degrees; shallower than that and rain will not clear dust properly, steeper and you lose winter sun.

Building the Frame and Base

A cedar greenhouse is only as good as its foundation. Set it on a level concrete strip or paving slabs, with a damp-proof course between the base and the timber sill. This prevents rising damp from wicking into the end grain, which is where rot usually begins. For a period property, a low brick plinth makes a handsome and practical base – use lime mortar if the house is old, to allow movement.

Good joinery is the heart of rural craftsmanship. Traditional mortise-and-tenon joints, pegged or secured with stainless steel screws, will outlast screwed butt joints many times over. Stainless steel is essential near glass and damp; ordinary steel will rust and stain the cedar. Allow a small expansion gap at each glazing bar, and bed the glass on putty or dry-glaze with silicone, depending on your preference. A well-built cedar frame should need nothing more than an annual check of fixings and a fresh coat of oil every two or three years.

Ventilation and Summer Cooling

Overheating is the most common cause of failure in a British greenhouse. Even on a mild May day, an unvented glasshouse can hit 40°C, which will cook tomatoes, scorch cucumbers, and stress young seedlings. Include both roof vents and low-level louvre vents. Roof vents let hot air escape, while louvres draw cool air in from near the ground. Aim for at least one roof vent per 2.5 metres of ridge, and a louvre on the opposite side to create cross-flow.

Automatic vent openers, powered by wax cylinders or solar motors, are a wise investment. They respond to temperature without you having to be there, opening at around 18°C and closing as the evening cools. For summer, add shading: a roller blind on the south side, a coat of shade paint, or horticultural netting. Damping down the floor on hot mornings raises humidity and cools the air, which suits most crops. If you grow tomatoes, aim to keep the daytime maximum below 28°C and the night minimum above 10°C.

Practical Layout and Year-Round Use

Inside, plan for productivity. A central path of brick or gravel, wide enough for a wheelbarrow, with staging along the south side and a deep border on the north. Staging made from cedar slats allows air to circulate around pots and prevents water pooling. Fit a water butt to the downpipe – rainwater is better for plants than tap water, and it saves on mains use during dry spells.

• Staging: cedar slats, not solid boards, so air moves freely.
• Watering: a water butt linked to the downpipe, with a tap and hose.
• Path: brick or gravel, wide enough for a wheelbarrow.

In winter, insulate with bubble wrap or fleece curtains on the north side, and use a soil-warming cable or a small electric heater only as a last resort. A max-min thermometer will tell you what your greenhouse is really doing. Finally, keep a note of what works: which varieties cropped early, where the cold spots are, how the vents behaved in a gale. A cedar greenhouse, well sited and well vented, will serve a productive garden for fifty years or more – and look quietly handsome against a period backdrop all the while.