Entry 17 · Windows & Glazing
Growing plants at a window and in a conservatory
How window glass, humidity and ventilation affect herbs and pots on sills and in conservatories, with practical checks for healthy indoor growth.
Growing herbs on a window sill or in a conservatory works, but the glass changes the conditions more than most growers expect. Modern double glazing filters out a large share of ultraviolet light, reduces the light that reaches leafy plants in winter, and traps both heat and moisture in a way that single panes never did. A healthy pot of basil or rosemary behind sealed units therefore depends on three things being managed together: light levels, humidity and air movement.
The Royal Horticultural Society publishes practical guidance on growing herbs indoors and on house plant care generally, and its advice on light and watering lines up with what experienced conservatory growers report: the failure point is rarely the plant, it is the corner of glass it sits behind.
01 What does double glazing do to light for plants?
Glass transmits visible light well, which is the part of the spectrum plants use most for photosynthesis, but it blocks almost all UVB and a significant share of UVA. That matters less for herbs than for seedlings, because aromatic plants such as basil, mint and rosemary rely mainly on visible and near-infrared light. The more serious problem is the reduction in total light, not the spectrum. A sealed unit has two panes and often a low-emissivity coating, and each layer takes a small share of the energy passing through. In December and January, a south-facing sill in England may deliver only a few hours of usable light, and a north-facing one almost none.
There are practical consequences. Herbs grown behind glass in winter tend to stretch, with long gaps between leaves and thin stems, a condition growers call etiolation. Potted chillies started on a sill in January will germinate but then stall. Anyone serious about leafy growth through the winter months either moves pots to the brightest available opening or adds a small full-spectrum grow lamp on a timer. Position matters too: light arriving at a low angle through a vertical window hits the plant from the side, so pots should be turned a quarter rotation each week to keep growth even.
A secondary effect of low-emissivity coatings is worth noting. Coated glass reflects part of the far-red and near-infrared radiation back. Plants sense the ratio of red to far-red light to judge shade, and a windowsill behind coated units can register as shadier than the raw daylight hours suggest. Growers who notice leggy, leaning basil on a bright sill are often seeing this effect rather than a lack of sun.
02 How hot and humid does a conservatory get behind sealed units?
Very, and quickly. A glazed structure behaves like a closed car in April sunshine: air temperature can climb from 15 to over 30 degrees Celsius within an hour, then fall fast after dusk. Sealed units with trickle vents closed make this swing worse, because there is no slow air exchange to buffer it. Herbs tolerate warmth, most of them originate in the Mediterranean, but they do not tolerate a swing of fifteen degrees in a few hours combined with a cold root zone at night.
Humidity follows the same pattern. Pots release water vapour continuously, and in a sealed conservatory the relative humidity can sit above 70 percent for hours. That is comfortable for ferns but risky for basil, mint and chilli plants, whose leaves stay wet with condensation in the evening. Persistent leaf wetness is the main trigger for grey mould, botrytis, and for the fungal spots that spread quickly across a crowded shelf of pots. A hygrometer costing a few pounds answers the question directly: if readings stay above about 65 percent for most of the day, the air needs to move or the number of pots needs to fall.
Ventilation is the lever that controls both temperature and humidity at once. Trickle vents, roof vents and an opening door should be used in sequence: trickle vents for constant background exchange, a roof vent or high window to let warm humid air escape by stack effect, and a door cracked open on sunny days above roughly 20 degrees. Where the structure allows no opening, a small desk fan on a timer, running for a few hours in the middle of the day, keeps air moving across the leaf surfaces and stops condensation settling. Detailed step-by-step protocols for setting up ventilation, ducting and timers in enclosed growing spaces are set out in the Italian technical guide panicogrowshop.it, whose grow room and grow box pages cover fans, thermostats and dehumidifiers in a way that translates directly to conservatory practice.
03 Which herbs suit a windowsill, and which need more?
Not all pot herbs behave the same behind glass. Basil wants warmth, steady light and regular picking; it sulks below about 15 degrees and despises cold draughts from a leaky frame, so it belongs on an inner sill away from a door. Rosemary and thyme prefer dry roots, gritty compost and as much direct sun as the window gives; they fail on shaded sills not from cold but from low light and soggy soil. Mint is the most forgiving, tolerating lower light and higher humidity than any other common kitchen herb, which makes it the natural choice for a north-facing window. Chillies need the strongest light of the group and fruit poorly behind glass in an English winter without supplementary lighting. Small courgette plants raised indoors for an early start should move outside or into the brightest conservatory position as soon as they have two true leaves, because their light demand rises fast.
Watering should follow the plant and the season, not a schedule. From November to February, growth slows and compost behind a cool window can stay wet for a fortnight. A finger pushed into the surface, or the weight of the pot lifted, tells more than any calendar. Pots standing in outer trays of trapped water are the classic cause of root rot on a sill, because the water has nowhere to go in a sealed room.
04 Does the window frame and glazing condition matter for plants?
It matters more than most guides admit. A draughty sash or a failed sealed unit creates a microclimate: cold air pours down the inside face of the glass at night, dropping the leaf temperature of any pot pressed against it several degrees below the room. Herbs touching the pane on a January night can suffer chilling injury even in a heated room, visible as dark translucent patches on basil leaves within days. The practical fix is spacing: keep foliage a few centimetres back from the glass, and close curtains or blinds behind pots only if they do not trap the plant against the cold surface.
Condensation on the inside of the pane is a diagnostic signal. If a window used to run with water and no longer does after an upgrade to double glazing, the room has become more airtight and humidity is now accumulating in the air rather than being vented through the frame. For the grower, that means the ventilation routine, not the watering can, is the first thing to adjust. It also means checking that trickle vents installed with the glazing are actually open, since many are taped shut after fitting and forgotten.
A sensible seasonal rhythm ties it together. In spring and summer, an ordinary south or east window grows basil, mint and chives with no equipment at all. From late October to March, expectations should drop, pots should be fewer, watering lighter, and either a lamp or the brightest conservatory position brought into use. Light, humidity and airflow are three dials on the same panel: turn one without the others and the plant shows it within a week.