Prick out seedlings before they get too crowded and then to pot them on as individual plants as soon as they are large enough.
Make sure you give greenhouse plants more space as they put on new growth. This will help to prevent disease, and to contain early pest infestations.
Check if plants need watering at least every few days and seedlings will need daily attention.
A minimum of 5°C (41°F) needs to be maintained to ensure that fuchsias, pelargoniums and other tender plants being killed by the cold. Tropical plants require a higher temperature of at least 12°C (54°F) which will require a greenhouse heater.
If weather turns warm apply a light covering of shade paint of the greenhouse panes to prevent plants becoming scorched by bright sunshine. Alternatively, a sheet of newspaper, or some shade netting will work fine as temporary shading for seedlings and young plants.
Temperatures can still drop at night. Seedlings in particular object to wide temperature fluctuations and a heated propagator, or fleece cover could be a cost effective answer if your greenhouse is mostly full of hardy mature plants.
If the weather is warm and sunny, you may need to start damping down the floor of the greenhouse in the morning by splashing water over it to increase humidity levels.
Open vents and doors during warm days.
Brush up fallen compost and debris, and pick off dead leaves from plants. This will help prevent pests and disease spreading.