Should I quarantine a new plant before putting it with the others?
Where a new plant's pests actually come from, the two-week check that catches them, and the specific things to look for before it joins anything else.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 3 min
Short answer
Yes for houseplants and for anything joining a group of pots, and it costs almost nothing. Keep a new plant apart for two to three weeks, inspect leaf undersides and the compost surface weekly, and knock it out of its pot once to look for grubs. Most introduced infestations arrive on a plant that looked perfectly clean in the shop.
Nurseries and garden centres grow plants close together in warm, still, sheltered conditions with no predators — the same conditions that let a pest population double quietly without being obvious on any individual plant. A new plant is the single most common way red spider mite, mealybug, scale and vine weevil enter a collection.
Step by step
- Inspect it before you buy it.Turn two or three leaves over. Look at the growing tips, the leaf axils and the underside of the midrib. Check the compost surface for fungus gnats, and look at the drainage holes for roots and for anything moving. Put it back if you are unsure.
- Keep it separate for two to three weeks.A different room for a houseplant, or a corner of the garden away from the pot group. Not touching another plant — most indoor pests spread by walking, so leaves that touch are a bridge.
- Check the underside of leaves weekly.You are looking for fine webbing at the shoot tips, white fluff in the leaf joints, brown limpets along the midrib, stippling, and anything sticky. A hand lens helps enormously and costs very little.
- Knock it out of the pot once and look at the rootball.Slide the pot off over a sheet of newspaper. You are looking for fat white C-shaped grubs with brown heads, which are vine weevil, and for white powdery patches on the roots, which are root mealybug. This one check has saved a lot of collections.
- Consider repotting into your own compost.For a houseplant, replacing the nursery compost removes gnat larvae, weevil eggs and any root pest in one action. Do it in growing season, not in midwinter, and expect the plant to sulk for a fortnight.
- Wash the leaves before it joins the others.A shower or a hose over both surfaces of every leaf removes a large proportion of mites and early aphids mechanically. Let it dry properly afterwards.
- Watch particularly closely if it came from a plant swap or a friend.Home-grown plants carry more pests than commercial ones, not fewer, because they have not been through a nursery's spray programme. The same applies to anything bought at a car boot or a charity plant stall.
- Do the same for cut flowers and for pots brought in for winter.Bought cut flowers routinely carry aphids and thrips onto houseplants nearby, and a plant coming indoors for winter brings whatever it acquired outdoors into a warm room where it can breed all year.
Tips
- Two weeks catches most things because it covers one generation. Three weeks is better for mealybug and scale, whose crawlers are the only visible stage.
- Yellow sticky traps in the quarantine area are a cheap early-warning system for fungus gnats, whitefly and winged aphids.
- If you find something, treat during quarantine. Treating one plant in isolation is a completely different job from treating a whole windowsill.
- Outdoors, quarantine matters less for general pests and a great deal for box plants, where box tree caterpillar and box blight both arrive on new plants.
Common mistakes
- Standing the new plant next to the others "just for now" — Now is when the crawlers move. Spider mite and mealybug cross between touching leaves within days.
- Assuming an expensive plant from a specialist is clean — Specialist nurseries are the commonest source of specialist pests, because they grow large numbers of one susceptible thing.
- Only checking the leaves — Vine weevil, root mealybug and fungus gnats are all in the compost, and none of them shows on the foliage until the plant is already failing.
Questions people ask
Do I need to quarantine outdoor plants too?
Less strictly, because outdoors there are predators and weather. Still check the rootball of any container-grown plant for vine weevil grubs before planting, and inspect box, hedging and fruit particularly carefully.
Where should quarantine actually be?
Anywhere with reasonable light that is not adjacent to other plants — a spare room, a bathroom windowsill, a porch, or a bench at the far end of the garden. Light matters, because a plant declining in a dark corner is harder to read.