How much and how often to water plants always puzzles the beginner, and there is no definite rule to follow. No one amount of water can be said to be right for pot plants, bench plants, or seedlings. Types of potting and benching soils; the amount of sunshine, humidity, and air circulation; the season of the year; and the location of the greenhouse are all factors in developing a watering program conducive to steady, healthy growth and bloom. Experience has produced some helpful fundamental practices, but don't be so influenced by these that you are afraid to depart from a schedule—this would be a mistake. Theoretically, water should be applied in the morning, preferably not later than noon, and on a rising temperature, because when you water plants there is a sudden drop in temperature. This causes moisture in the air to condense, forming droplets of water on foliage and flowers. If there is not enough time before darkness for this moisture to evaporate, conditions conducive to disease are created. But this is a general rule, so be guided accordingly.
On occasion, if you find your seedlings drying out, or your primroses and hydrangeas wilting in the afternoon, have no qualms about watering them at once. You may also find that some plants always need extra water, and at odd times, to keep them healthy. If you go to business during the day, you may sometimes have to skip morning watering. On checking your greenhouse in the evening, if you find plants looking fairly dry and you doubt that they should be left until the next morning for watering, by all means water them, even if it is night time. But be careful not to splash foliage, and don't be heavy-handed; this is only a tide-over watering to be completed the next morning.
Plants absorb water from soil by means of their roots; some of the water is used in food manufacture, the rest is transpired (given back to the air) through the foliage. Too much water is to be avoided, particularly for seedlings and new cuttings, for these have very fine root hairs that quickly die if they are compressed by wet soil from which all oxygen has been eliminated. That is why sharp sand, vermiculite, or sphagnum moss are recommended for seeds and cuttings. The nature of these mediums prevents water pressure from packing themselves around tiny new roots. Misting or light spraying of cuttings is also beneficial. The mist replaces moisture transpired through the leaves, cools plants by evaporation, and helps roots become established before they must take on the job of completely supplying plants with both water and food.
Keep in mind that soils differ in consistency. A loose, sandy soil holds moisture only a short time. Soils that are heavy or clayey pack and hold water longer, even to the point of sogginess. Obviously, plants in sandy soil require more water to become fully moist while those in heavy soil require less water. Plants in clay pots require more water than those in plastic containers because clay pots themselves absorb water. Plastic is not water absorbent and it keeps air from reaching the soil to dry it out.
On sunny days or during the period when the heat is turned on, repeated watering may be necessary because heat makes soil dry out quite rapidly. During cool rainy spells, soil dries out slowly and less water is needed. As you see, only suggestions, not rules, can be given to help with your watering schedule; much depends upon circumstances. Like the rest of us, you will have to rely upon a certain amount of trial and error to establish a satisfactory procedure. In any case, it is better to err on the side of too little than too much. If a plant dries out slightly between watering, even to the point of wilting, it is easier to perk it up and keep it healthy than to revive it if heavy hosing has packed soil around roots and cut off their supply of oxygen, which often proves fatal to the plant.
Be patient; use common sense. You will be surprised how quickly you will "get the feel" of the soil and overcome your qualms about watering too much or too little. And once you do get the feel of it, you will find it takes less time to take care of all your plants.