One pot is enough
A greenhouse in winter is a closed, warm, still, predator-free room. It is a poor environment for the beneficial insects that spent the summer eating your pests outdoors, and an outstanding one for the pests themselves. Whitefly, aphids, red spider mite, mealybug and scale all breed through winter under glass at a rate they never manage outside.
Which means the arithmetic is unforgiving. A single unnoticed colony on a single pot, carried in during October, has until March to multiply in ideal conditions with nothing eating it. By spring it is not on one plant, it is on all of them, and you are starting the growing season fighting an infestation instead of sowing.
The protocol below takes about twenty minutes a plant. Against a whitefly-covered February it is the best twenty minutes you will spend this autumn.
The move-in is a gate, not a shuffle.
Stage five of the fall changeover sequence puts move-in after cleaning, sealing, insulating and heating for exactly this reason. You have just spent weeks producing a clean, sealed space. Everything crossing that threshold should have been inspected, and nothing should cross it because you ran out of daylight.
The inspection, in order
Do this outside, in good light, ideally with a hand lens. Work through every plant in the same order so you do not skip steps when you get tired at plant number thirty.
- Undersides of leaves. Where nearly everything lives. Whitefly adults fly up when disturbed; their scales and eggs sit still on the underside. Aphid colonies cluster on the underside near veins.
- Growing points and new shoots. Soft new growth is what aphids want. Check the tip of every stem.
- Leaf joints and stem crevices. Mealybug shows as white cottony masses; scale as small immobile brown or grey bumps that do not brush off.
- Fine webbing. Between leaves and at shoot tips, indicating red spider mite. The mites themselves are near-invisible; the webbing and a fine pale speckling on the upper leaf surface are the giveaways.
- The top inch of compost. Fungus gnat larvae, and the small white springtails that indicate persistently wet compost.
- The pot itself. Outside surface, rim, and underneath — slugs, snails, woodlice and vine weevil adults all travel this way.
- The drainage holes. Roots growing out mean the plant is potbound; anything living in there comes in with it.
The knock test
Hold a sheet of white paper under the foliage and tap the plant sharply. Anything that falls off is visible against the white, and this catches mites and thrips that a visual inspection misses entirely. Ten seconds per plant.
Triage: in, hold, or out
Clean. Nothing found on inspection or knock test. Repot or top-dress if needed, wash the outside of the pot, and it goes into the greenhouse.
Treatable. Something found, but limited and identifiable. Treat, then hold in quarantine for at least two weeks and re-inspect before it joins the others.
Not worth it. This is a real category and people avoid it. A heavily infested plant, a common variety easily replaced from cuttings, or something you kept out of habit rather than affection — the risk it carries into a closed winter house outweighs its value. Take a clean cutting if it matters to you and compost the parent.
Running an actual quarantine
Quarantine means physically separate, not "the far end of the same bench". Pests move between touching foliage and fly further than the length of a greenhouse.
Options that work: a cold frame, a porch, a spare room, or a section of the greenhouse divided by a sheet of fleece or film to bench height and above. Two weeks minimum, three is better, because it lets a generation of eggs hatch and reveal itself.
During quarantine, hang a yellow sticky trap next to the plant. Whitefly, fungus gnats and thrips are attracted to yellow, and a trap that stays empty for two weeks is real evidence rather than a hopeful glance.
Treatment before entry
The aim is to send in a clean plant, not to start a chemical program indoors.
- Physical removal first. A firm spray of water on the undersides removes a large share of aphids and mites. Wipe scale and mealybug off by hand with a cloth or a cotton bud.
- Remove infested growth. Cutting off two heavily colonised shoots is faster, more reliable and less disruptive than treating the whole plant.
- Soap or oil sprays where you need them. Cover the undersides thoroughly, because these work on contact and miss anything they do not reach. Repeat after a week to catch newly hatched eggs.
- Repot suspect compost. If the top inch is alive with gnat larvae, the simplest fix is fresh compost and a cleaned pot.
- Follow the label on anything you buy, particularly on plants you will eat from. Do the treating outside, before entry.
Preparing the plant while you are at it
The move-in is also the annual maintenance moment for these plants, and doing it now saves handling them twice.
- Cut back leggy summer growth. Less foliage means less to keep alive in low light, and better air movement around the plant.
- Remove every yellowing leaf and spent flower — the material gray mold starts on, as covered in autumn condensation and gray mold.
- Top-dress or repot into fresh compost. Do not feed; you are not trying to push growth into winter.
- Lose the saucer, or empty it. Standing water under a pot all winter is root rot waiting to happen.
- Space plants further apart than feels right. Crowding in a closed house is how everything spreads.
The kit
Yellow sticky traps
$ · Quarantine monitoring and winter early warning · buy a pack, use them all
Whitefly, fungus gnats and thrips are drawn to yellow, which makes these both a monitoring tool and a modest control. A trap next to a quarantined plant that stays empty for two weeks is real evidence. Leave a few up all winter as an early warning system.
Hand lens
$ · Inspection · mites and scale are not visible without one
Red spider mite is at the edge of what the naked eye resolves, and scale insects look like part of the stem until magnified. A basic 10x loupe turns a guess into a diagnosis and costs less than one replacement plant.
Insecticidal soap or horticultural oil
$ · Pre-entry treatment · contact action, cover undersides
Contact treatments that work on soft-bodied pests and break down quickly. They only kill what they touch, so coverage of the leaf undersides is everything, and a second application a week later catches newly hatched eggs. Apply outside, before the plant comes in, and follow the label on edibles.
The first month inside
Quarantine ends but vigilance does not. Check sticky traps weekly through November and December, and inspect leaf undersides once a fortnight. Populations that start small in a warm house become visible around January, and by then they have been building for two months.
Catching a small outbreak in November means removing a few leaves. Catching the same outbreak in February means treating the whole house repeatedly, at exactly the point in the year when your plants are weakest and your ventilation options are worst.
Greenhouse Guide earns commission on qualifying purchases made through the links above, at no additional cost to you. We link to product categories rather than single items because stock and pricing in this category change week to week — compare current listings and buy what fits your structure.
Common questions
How long should I quarantine plants before bringing them into the greenhouse?
Two weeks minimum, three if you have the space. That interval lets a generation of eggs hatch and reveal itself, which a single inspection on the day cannot. Keep quarantined plants physically separate, not just at the far end of the same bench.
What pests come in on container plants in autumn?
Whitefly, aphids, red spider mite, mealybug, scale and fungus gnats are the common ones, along with slugs, woodlice and vine weevil travelling on and under the pot. Check leaf undersides, growing points, leaf joints, the top inch of compost and the outside of the pot.
What is the knock test?
Hold a sheet of white paper under the foliage and tap the plant sharply. Anything that falls is visible against the white, which catches mites and thrips that a visual inspection misses. It takes about ten seconds per plant and is the highest-yield part of the check.
Should I spray plants before bringing them inside?
Only if the inspection found something. Physical removal first — a firm water spray on the undersides, wiping scale off by hand, cutting out heavily infested shoots. Where a contact spray is needed, apply it outside, cover the leaf undersides thoroughly, and repeat after a week.
Is it worth bringing in a badly infested plant?
Usually not. In a closed winter greenhouse a heavy infestation will spread to everything, and the plant is often easily replaced or regrown from a clean cutting. Take the cutting if it matters to you and compost the parent rather than importing the problem.