The end of the dacha season and an empty greenhouse create the illusion that the battle against pests is over. However, according to the Royal Horticultural Society (RHS), it is precisely during this period that hotspots form in the structure, soil, and plant residues, and they will persist until spring. Specialists from Utah State University Extension and Penn State Extension emphasize that whiteflies, two-spotted spider mites, thrips, and aphids are able to survive the cold months in the crevices of the frame, stakes, old ties, and weeds. In spring, roughly from the end of March, the populations become active again, start feeding and laying eggs — and the gardener is forced to begin the season not with planting, but with emergency treatment.
Spider mites: a pest that is noticed too late
The two-spotted spider mite (Tetranychus urticae) remains one of the most unwelcome inhabitants of protected cultivation. Its size is extremely small, so colonies are visually detected only at the stage when characteristic light spots already appear on the leaves, the leaf blade begins to bronze, and a thin web is stretched between the stems and leaves. RHS explicitly points to the need to monitor this species in greenhouses in autumn. The mites' hiding places are the crevices in the frame, cracks in the seals, old stakes, and unremoved plant residues. If these elements are not removed in autumn, the mites will emerge from their shelters in spring and attack the first transplants put in the ground.
Whiteflies: a multi-generational inhabitant that knows no winter
The greenhouse whitefly (Trialeurodes vaporarum) is especially well adapted to the conditions of protected cultivation. Specialists from Utah State University Extension note that within a single growing season the pest is able to produce several generations, and in greenhouses its populations are maintained from year to year, effectively without interruption. The mechanism of damage is simple: larvae and adults suck cell sap from the plant tissues, as a result of which the leaves yellow, lose turgor, and drop prematurely. It is critically important that whiteflies are not limited to crop plants — they live just as successfully on weeds and "accidental" plants that have ended up in the greenhouse by chance. Therefore, after the end of the season, leaving any green shoots in the structure is unacceptable.
Thrips and aphids: a hidden threat in the axils and on the underside of leaves
Thrips, aphids, and other sucking pests are distinguished by the fact that they hide in hard-to-reach places: on the underside of leaves, in the stem axils, in the inflorescences. Penn State Extension gives a specific example of infection transfer between crops: aphids that overwintered on tomato residues move in spring to the next crop — lettuce or other greens. Thus, the problem arises "very early," even before the gardener has a chance to assess the condition of the plantings. Specialists emphasize that sanitary cleanup of the greenhouse when transitioning between crops limits the resources the pests need to survive and reduces the risk of their transfer to subsequent plantings. In this context, the phrase "I'll leave the bushes for just a little longer" means a guaranteed infection of the new crop.
Autumn cleanup: the step-by-step minimum recommended by RHS and university extension services
RHS calls autumn one of the best times for cleaning the greenhouse. The recommended minimum: remove all plant residues, old ties (including plastic straps and twine), stakes, weeds, and any organic waste; thoroughly wash the structure. For ordinary cleaning of the glass and frames, warm water is sufficient — special chemical agents, according to RHS, are not mandatory. However, it is precisely the mechanical removal of shelters, rather than chemical treatment, that is the key factor in reducing the number of overwintering populations. Penn State Extension adds: the fewer pests and hiding places remain in the greenhouse in autumn, the lower the probability of encountering them immediately after planting new transplants in spring. Ignoring this cleanup effectively means that the gardener is creating an "incubator" for pests for the entire cold season.
Practical conclusion: do not wait for spring
The combined recommendations of RHS, Utah State University Extension, and Penn State Extension come down to one simple rule: if spider mites, whiteflies, thrips, or aphids were detected in the greenhouse during the summer, the autumn cleanup should be carried out with special care. Leaving old bushes, ties, and weeds "for later" means guaranteedly starting the spring season with a fight for the harvest rather than growing it. Sanitary treatment of the greenhouse in autumn is not an optional procedure but a mandatory element of protected-cultivation agronomy.
