Unlike outdoors! Keeping plants grown under cover supplied with sufficient moisture to meet their needs can often be quite daunting, particularly when you consider; in very hot weather, they may need to be watered twice a day.
Invariably this task has to be done manually!
Luckily there are a few things you can do and use to make things easier.
For example; placing pots on capillary matting set in shallow gravel/water trays allows the plants to absorb water when they need it.
Damping down the floor of the greenhouse with water every morning will increase humidity and reduce drying out.
Good shading and ventilation will also reduce rates of evaporation.
Investing in an automatic drip system with a built-in reservoir, should take care of most of the necessary plant irrigation required, particularly at holiday time.
Note;
Bedding plants planted outdoors ; need regular watering in dry weather, and are better watered either first thing in the morning or during the evening when the sunlight is not as strong.
Planting out at wider spacing can reduce the need for water; the closer plants are the more water they need.