The £20 Question: Do You Need Bottom Heat to Root Cuttings in a Cold Shed?

The shed at the end of my allotment has one redeeming feature in November: it keeps the rain off. It does not keep out the cold. The single-glazed window is more of a suggestion than a barrier, and the thermometer inside regularly sits within a degree or two of the outside air. This is where I am running a winter cuttings trial, because most of us do not have a heated propagator and many of us do have a roll of bubble wrap.

The question is straightforward. If I take hardy-ish shrub cuttings in late autumn and stick them in a cold shed, does wrapping the pots in bubble wrap give me anything close to the performance of a cheap heat mat? Or is bottom heat in a different league entirely? The trial runs from the first week of November 2026 into January 2027, with rosemary, honeysuckle and privet as the test subjects.

The three rigs

Everything sits on the same bench, under the same gloomy light, in the same shed. There are three treatments.

1. The control

Standard 9cm pots, no wrap, no heat. Just the ambient shed temperature and whatever thermal mass the bench provides. This is the honest baseline: what happens if you do nothing clever at all.

2. The bubble-wrap pot

Each pot is wrapped around the sides with two layers of bubble wrap, with the bubbles facing inward. The pots stand on a scrap of polystyrene to stop the bench pulling heat out of the base. There is no lid over the top and no sealed plastic bag. The idea is insulation, not a mini rainforest.

3. The budget heat mat

A 21-watt seedling heat mat, the sort that costs about £20, with a cheap probe thermometer pushed into one pot to monitor root-zone temperature. It is not a laboratory thermostat, and the mat has a modest warm spot rather than an even blanket of heat, but it is the kind of kit a UK gardener actually buys.

Each treatment gets ten cuttings of each species: rosemary, honeysuckle and privet. That is 90 cuttings in total. All are nodal cuttings, roughly 8-10cm long, trimmed just below a node, with the lower leaves removed. They go into a 50:50 mix of multipurpose compost and perlite. No rooting hormone, because I want the comparison to be about temperature and insulation, not chemical assistance.

What I am measuring

This is not a controlled lab trial. It is a shed trial with a notebook, and I am treating it that way. Still, the measurements are chosen to answer practical questions.

• Root-zone temperature, logged hourly with a cheap sensor in one representative pot per treatment.
• Root emergence at four, six, eight and ten weeks, checked by gentle tug and then by tipping out a few pots.
• Condensation score, from dry to dripping, recorded each morning.
• Failure rate, including mould, blackened stems and complete mush.
• General vigour of any roots produced, because a single brown thread is not the same as a healthy root system.

The first fortnight, as of 8 November 2026

The trial is only just underway, so these are early notes rather than verdicts. The heat mat is already doing what you would expect: root-zone temperatures are hovering around 20-24°C, even when the shed air drops to 7°C overnight. The bubble-wrapped pots are warmer than the control, but only by a couple of degrees, typically 8-14°C. The unwrapped control is sitting at 6-12°C. That gap matters, but it is not the same as bottom heat.

Condensation is the interesting part. The bubble wrap itself does not seal in moisture, but it does slow the drying of the compost and keep the pot sides warmer. The result is a slightly more humid microclimate around the stems. That could help. It could also encourage mould if I get lazy with ventilation. I am already opening the shed door for an hour on dry days and wiping the inside of the window.

Why insulation is not the same as heat

Bubble wrap is often described as if it generates warmth. It does not. It traps air and slows heat loss. In a cold shed, that means the pot cools down more slowly and swings less wildly between day and night. A heat mat, by contrast, adds energy. It can hold the root zone at a temperature that encourages cell division and root initiation even when the air is cold.

That distinction is the whole experiment. Cuttings do not root on a schedule; they root when the tissue around the cut end has enough warmth, oxygen, moisture and hormonal signalling to form adventitious roots. Rosemary is the fussy one here. It is slow and rot-prone in cool, wet conditions. Honeysuckle is more forgiving. Privet is about as close to a sure thing as shrub cuttings get, which makes it a useful check on whether the method is working at all.

The decision rules

I am not looking for the heat mat to lose. I am looking for a sensible cheap option. So here is how I will judge the bubble wrap.

If the bubble-wrapped pots produce at least 60% of the heat mat's rooted cuttings by week eight, with no serious mould problem, I will call it a useful low-cost method for easy species. If they root less than a third as many, or if condensation causes more rot than the control, it is a false economy. The heat mat can be reserved for the difficult stuff: rosemary, woody herbs and anything that sulks below 15°C.

The cheap winter method I expect to recommend

Whatever the final numbers say, a few principles are already clear. Insulate the base as well as the sides. Do not seal the top. Ventilate daily. Keep the compost barely moist, not wet. Use a heat mat with a thermostat if you can stretch to one, because a mat without control can cook a small pot on a mild day. And do not expect bubble wrap to turn a freezing shed into a propagator.

For UK gardeners without bottom heat, the realistic approach is to wrap the pots, stand them on polystyrene, group them together for mutual shelter and choose the easiest species first. Rosemary is not the plant to learn on in November. Privet and honeysuckle will tell you whether your setup is basically sound. Then, if you want to push into trickier cuttings, spend the £20 on heat rather than another layer of bubble wrap.

The trial continues. I will be back with root counts, mould scores and a final verdict once the January check is done.