Bagged or Bare? The November Windowsill Test That Splits the Clear-Bag Habit

On 8 November 2026, I lined up twelve small pots on a spare-room windowsill in south Manchester. Half had a clear freezer bag over them, propped on two skewers and secured with an elastic band. Half sat open to the room. The question was simple: does the standard plastic-bag trick actually help cuttings root, or does it just create a warm, wet tent for mould?

This was not a polished laboratory trial. It was a windowsill experiment with six pots per treatment, two cuttings per pot, and a notebook. But it was enough to expose the shape of the argument, and to make me rethink the ritual of bagging every cutting I take in November.

The setup

I used three forgiving houseplants: Epipremnum (pothos), Tradescantia zebrina and spider plant. All are easy from stem cuttings, which makes them good test subjects: if a treatment fails on these, it will fail on most things. The room was unheated for most of the day, around 17–19°C, dropping to 14°C at night. The windowsill got weak November light, no direct sun. There was no bottom heat and no rooting powder.

The bagged pots were sealed at the rim but opened for ten minutes each morning. That detail matters. A fully sealed bag is not a humidity dome; it is a gas trap. The open pots were left bare, watered when the compost surface dried, and turned every few days.

I tracked four things: condensation, mould, leaf loss and first roots. I did not tug the cuttings to check. I waited until roots appeared at the drainage holes or until a gentle lift of the cutting met resistance.

What the bags did

The bagged pots condensed within two days. That is normal: warm, moist air hits cooler plastic and water drops out. The problem was not condensation on the bag. It was condensation on the leaves, which stayed beaded for hours in the still air.

Measure	Bagged	Open
Condensation	Heavy by day 2; leaves wet for long spells	None
Mould	3 of 6 tradescantia pots by day 10	None
Leaf loss	1 leaf	5 leaves
First roots, pothos	Day 12	Day 16
First roots, tradescantia	Day 9	Day 10
First roots, spider plant	Day 18	Day 21

The pattern was not a clean win for either side. Bagged pothos and spider plant rooted earlier, and the open cuttings lost more leaves. But the bagged tradescantia were a mess: soft stems, fuzzy grey mould and one pot that smelled like a forgotten salad drawer. The open tradescantia rooted almost as fast and stayed clean.

That is the first lesson. Humidity is not a single variable. It helps a leafy cutting stay turgid, but it also slows evaporation from wet leaves and reduces air movement. In a warm room, that is a recipe for disease.

The science bit: humidity, gas and mould

A cutting has no roots, so it cannot replace the water it loses through its leaves. Most of that loss happens through stomata, the adjustable pores on the leaf surface. When the air is dry, the vapour pressure deficit is high and water leaves the leaf quickly. A bag raises the humidity around the cutting, lowers that deficit and slows transpiration. For a soft, leafy cutting on a dry windowsill, that can buy the days it needs to form adventitious roots.

But a bag does more than hold moisture. It also traps ethylene, the gas plants use to regulate ripening, senescence and leaf fall. Ethylene can build up in a sealed space and encourage the very leaf drop you are trying to avoid. A bag also limits carbon dioxide exchange. In November light, photosynthesis is already slow, and a sealed bag does not improve it.

Then there is the disease side. Botrytis and damping-off fungi love still, humid air and wet leaves. A bag over a warm windowsill is a small greenhouse with no vents. The condensation itself is not the enemy; persistent leaf wetness and stale air are. That is why the standard advice often says to open the bag for a few minutes a day. It is not fussing. It is ventilation.

When a dome earns its place

A humidity dome, whether it is a purpose-made propagator lid or a freezer bag, is useful when the cutting is leafy, the room is dry and the cutting is not yet rooted. It is most helpful in the first week or so, before roots take over water uptake. It is least helpful when the air is already warm and still, when the cutting is crowded, or when the leaves are fuzzy or succulent.

I would use one for pothos, philodendron, ferns and other soft-leaved houseplants in a dry room. I would not use one for succulents, cacti, fuzzy-leaved plants such as African violets, or any cutting that is already rotting at the base. I would also avoid sealing it completely. A ventilated lid, or a bag with a corner open and a daily airing, gives you most of the humidity benefit with less mould risk.

The other rule is to wean. Once roots are forming, the cutting needs to adjust to room air. Open the vents wider over three to five days, or take the bag off for longer each day. Going from a sealed bag to a dry windowsill in one step is a shock.

The verdict

The plastic bag is not a myth. It is a blunt tool. It can speed up rooting in easy, leafy cuttings on a dry November windowsill, but it can also weaken stems and invite mould, especially in a warm room with poor air movement. The bag did not make bad cuttings root. It made good cuttings root a little faster and made damp cuttings rot a little sooner.

If you are taking cuttings this month, ask what the bag is doing. Is the room dry? Are the leaves soft? Can you ventilate? If the answer is no to ventilation, leave the bag off. A bare pot on a cool windowsill, checked every few days, is often kinder than a sealed plastic tent.