As crops finish and temperatures fall, there is a strong temptation to give everything a thorough clean-up. Clear every bed, cut every margin, spray anything still moving and start again in spring. Sometimes a clean break is exactly what a crop needs. But if we treat the whole nursery or growing site in the same way, we may remove the beneficial insects we spent the season introducing, along with the naturally occurring ones that arrived on their own.
Winter preparation is really a question of what should be removed, what should be protected, and where problems are likely to start again.
In a protected crop, begin with a proper walk before the old crop comes out. Where are the aphids, whitefly, thrips or mites now? Are they widespread, or concentrated around particular doors, vents, benches or rows? Are predators and parasitoids still present? Look at the crop as well as the traps: an empty trap does not tell you everything that is happening on a leaf.
Map the problem areas, even if it is only a marked-up plan and a few dated photographs. Record the pest, its severity, the crop stage, what you released or applied, and whether it worked. Come spring, that record tells you where to inspect first and where an early, targeted release may be more useful than treating the whole house equally.
Hygiene still matters. Old infested crops, contaminated growing media, weeds carrying pests, and debris around production areas should be dealt with. Benches, propagation spaces, tools and other sensitive areas may need thorough cleaning and disinfection between crops. The point is to make that decision according to the pest or disease risk in each area. A heavily infested glasshouse and a sheltered habitat outside it do not need the same treatment. In some cases, particularly where pests can survive in the structure or on remaining plants, a much more complete clean-out will be justified. (horticulture.ahdb.org.uk)
The same thinking applies to sprays. A broad-spectrum clean-up spray can be attractive at the end of a difficult season, but it can also disrupt the natural enemies that would otherwise help with the next one. Before spraying, ask what pest is present, where it is, whether it will survive the winter in that crop or structure, and what effect the treatment will have on the beneficials already there. Where treatment is needed, make it as well targeted and compatible with the biological control programme as the situation allows. (gov.uk, horticulture.ahdb.org.uk)
Perennial crops make this particularly relevant. Strawberries, soft fruit, orchard crops and long-term nursery stock retain living structure from one season to the next. That can carry beneficials forward, but it can carry pests forward too. Check crowns, bark, supports, old foliage and sheltered corners. Do not assume that because activity slows down, the pest has gone. Equally, do not assume every insect found in a sheltered place is a problem.
Annual outdoor crops have a different opportunity. Once the crop is removed, the surrounding habitat can still support natural enemies through winter. Hedgerows, grassy margins, flowering strips and undisturbed patches provide shelter and, at different times of year, food. Cutting or clearing every margin at once leaves very little continuity. Where practical, manage them in sections and leave some stems, tussocky grass, leaf litter and sheltered ground until spring. Keep those areas away from places where crop hygiene or disease management requires a clean surface. (rhs.org.uk, gov.uk)
If the site has very little winter shelter, this is a good time to create some. It need not be elaborate. A retained grassy strip, a managed hedge base, a small area of standing stems or a suitable patch of leaf litter may be more useful than an insect hotel placed for appearance. Think about where beneficials can survive and how they will reach the crop when activity resumes.
Introduced beneficials need a little realism in the plan as well. Some may persist in a particular crop and environment; others will not reliably carry through a cold winter or a complete crop turnaround. Record what you released, when and where, then check what is actually present as the new season starts. Winter habitat can support the wider beneficial community, but it does not replace monitoring or a planned spring release where one is needed. Research in raspberries, for example, found different overwintering survival between two predatory mite species, which is a useful reminder not to treat all beneficials as interchangeable. (horticulture.ahdb.org.uk)
For me, the most valuable winter job is an audit. Which pest caused the most trouble? Where did it first appear? Did we spot it early enough? Which beneficials established, and where did they struggle? Was a spray or a clean-out necessary across the whole site, or only in one part of it?
Those answers turn winter work into a better spring strategy. Clean the places that need cleaning. Remove genuine reservoirs of pests and disease. Keep useful habitat where it is safe to do so. Then start next season with a map, a monitoring plan and some knowledge of what was already living on the site.
Biological control does not begin with the first box of predators arriving in spring. Some of its most useful decisions are made now.