Getting seedlings off to a good start in spring means providing them with the best possible growing environment. As such, you should always start with a thorough deep cleaning. This not only helps rid the greenhouse environment of fungal pathogens but also helps eliminate the presence of insect pests and slugs. Many people living in temperate zones assume that insect and slug populations die out during cold winters, but these pests can survive in a greenhouse during the winter months.
Greenhouse Cleaning Basics
Choose a warm, calm day to clean your greenhouse. Not only will it be a more pleasant experience, but you'll be able to move any existing plants outdoors for the duration of the cleaning, allowing you to get to every nook and cranny in the structure. Be sure to get an early start so you'll have ample time.
After removing dead plant matter and other debris, scrub down all surfaces with a solution of mild detergent and warm water. Pay special attention to corners and other out-of-the-way places where insects and other pests like to hide. Algae and mildew often build up on greenhouse windows, so be sure you wash them thoroughly to remove all possible traces of them.
Disinfect Everything in the Greenhouse
Disinfect all surfaces such as benches, potting areas, and shelves using either a disinfectant specifically formulated for use in greenhouses or a solution of one part bleach and nine parts water. You should also disinfect pots, tools, hose nozzles, and anything else you use in your greenhouse. Disinfecting the floor should be the last thing you do before checking the outside of the greenhouse for holes, rips, tears, and any other repair and maintenance issues.
Replace the Soil
Potting soil loses nutrients over time and also provides a prime breeding ground for pests. This makes replenishing the potting soil is the last step in preparing your greenhouse for spring.
Also, check out this great guide on when to start planting.