In general, plants grown in a closed ecosystem like a greenhouse need more care and attention than plants grown out of doors. Plants in a garden can rely on rain for an occasional drink. The plants in your greenhouse, however, are entirely dependent on you for every drop that they receive. When watering the plants in your green house, you must be closely acquainted with the water requirement of each species as well as with the level of humidity in your green house.
Water your greenhouse plants early in the morning or late in the evening when temperatures are at their lowest.
Perfect your watering technique. Plants in a greenhouse should be watered with an easy stream of water like that supplied through a watering can. High pressure can cause water to splash. This spreads disease and encourages the growth of soil fungi. And when watering your plants, don't flood them immediately. Instead, give them a little bit at a time. Let each drink soak into the soil before giving the next. Stop when water runs out of the bottom of the pot.
Allow your plants to dry out. Although some plants demand moist soil, most thrive when their soil is allowed to dry out. Instead of watering your plants a few times each day, hold off until the top 2 inches of the soil are dry (unless the plant requires otherwise). Because the humidity level of greenhouses is so high, it is quite easy to over-water greenhouse plants.
Pay attention to the season. In summer, greenhouse plants will have to be watered more frequently than in spring or fall so check the soil more often. In winter, the water requirement of most plants drops off considerably. Furthermore, a damp cold environment in a greenhouse can lead to the growth of fungus.