Outdoors it's possible to grow most crops with a visit once a week, or even less. Under cover, unless you are ruthless with your planning and invest in automatic systems, you MUST visit more frequently, probably at least twice daily, and preferably more often. Outdoors we can often rely on nature to provide the water and change the air as our plants require. Under cover we are in charge and must do these important tasks not only well, but also with monotonous, unbroken, regularity.
If we are not careful we can make a rod for our own backs -especially if we have many young plants in pots and containers in a small greenhouse for then it may be risky to leave them untended for more than just an hour on a rapidly changing spring day when a cold cloudy start gets replaced by a scorching sun. Indeed most of us have too many different plants, at different stages of growth, in a multiplicity of sizes and types of container, to ever get most automatic watering systems to run without constant adaptation. Automatic systems are more suited to the professional with ranks of the same plants at the same stage and for those with mature specimens of known behaviour. Indeed I found the setting up and continual adjustment made one more work than doing the watering by hand.
The problem is the varying needs day by day of each different plant. It is hard for an experienced hand to judge the water needs of just one crop day by day in a changeable season. Given a host of different sorts at different stages, in different size containers with different composts and the personal touch and individual attention is really essential. There are few machines as sensitive as sticking your thumb in the compost. And our small greenhouses, and even polytunnels, vary widely in their daily conditions as even with some automatic heating and ventilation there are still the extreme vagaries of sunny yet chilly days for these to contend with. Generally of course on sunny days more water is needed, but not in those days immediately after repotting, or nipping out. And on colder days little is needed -except for those plants a little bigger than what they should be in too small a pot. These last will always need watering three times as often as others in an optimum size container.
Plants in pots have a very small amount of compost to draw on and if this dries out, or even approaches it, they wilt and serious damage is done to the final harvest. In fact if the plant does wilt irreparable damage is done, but some damage is done even before the wilt point. But far worse is being too wet as then the plants fail to grow and may even get root rots. We can't readily give larger volumes of compost in compensation because of the space limitations -especially in a propagator or a small greenhouse or coldframe. And also, even if we could, unfortunately excess wet compost not full of healthy roots may give off ammonia which can rapidly kill any roots growing nearby. This last effect is more common than you may imagine and is frequently the initial cause of seedlings failing in wet compost.
In the open ground roots can dive deep and wide and draw on large volumes of soil for their water -making a large quantity only slightly drier in the process. To acquire the same amount of water from far less compost the roots in pots have to work much harder, and in continually changing conditions. From water-logged immediately after watering to near parched dry again, somehow they have to find enough for the plant's needs. And don't forget there are manifold needs -some water 'sucked in' with nutrients is bound up in the plant as sap and cell fluids, and more is also there combined and converted into all the compounds making up the solid plant, and yet more is transpired off pumping the sap around the plant. If just slightly insufficient water is available the plant may look healthy, but grow slowly and can not do it's best for us. Most affecting the water consumption is the sunlight; with more light comes more growth and as more activity takes place so more water is needed. Heat produced by sunlight is a secondary factor, a max/min thermometer is essential as it tells you what is going on and whether your heating and ventilation is sufficient.
Under cover the water consumption is also altered by the ventilation, more air flow using more water. Indeed outdoors also, more wind equals more water lost both transpired and from direct evaporation -and often this may be excessive causing desiccation. Under cover we have a more closed system without such extreme wind losses but where unless we are careful the airflow may easily be insufficient and become stagnant and worse; damp. Once the air is full of water then it makes it difficult for the plants to transpire so their pumping system can't work easily and growth is hampered. Moulds and diseases thrive in such conditions -often starting during a cold phase these may just get worse as it warms up. The answer is more air; changing the damp humid air for dry air has an amazingly invigorating effect on plants as long as the air is not so cold as to chill them. In general the more air you can get moving over your plants the better -except few like to be in a cold draught.
And just as importantly but often overlooked is the carbon dioxide lacking in stagnant air under cover. The plants breathe it in and with sunlight convert it to oxygen extracting the carbon as their building block to combine with water. If we have insufficient ventilation then the plants don't get enough change of air to grow freely and may stop entirely. Of course it ought to be simple to ventilate -open everything up, but then during colder periods we may not be able to keep up the heat sufficiently as well. In commercial greenhouses bottled carbon dioxide gas is added enabling them to use less ventilation and to conserve heat. In the kitchen garden a home made carbon dioxide generator is yeast and jam or fruit juice in water in plastic bottles; these fizz for a fortnight or so before needing replacing.
However it is still better to ventilate as much as possible, Most greenhouses come with too few opening vents and we tend to further under utilise these. Unless it is cold and windy then all ventilation should be opened as soon each day as can be done without chilling the plants. If it gets hot and steamy and then we suddenly open up they get shocked and this often results in mildew or scorched leaves. Automatic vent openers are essential on at least a couple of vents! Ideally all the cover would be removable for the warmest days but this is impractical. (There have been greenhouses on rails that could be moved entirely on and off the crop!) Extra roof vents are the first priority to vent hot air, then extra side vents to let more air in as well as letting the hot out. The door is another option and should be opened all day whenever possible. (I've found it necessary to make a mesh screen door to keep out the birds and other larger vermin.) I've also joined two second hand houses together making a bigger volume with more vents and doors at each 'end'.
Often having under-opened rather late we also tend to leave the ventilation too open too long! It is best to shut down in the warmth of the mid to late afternoon so as to capture the last of the sun's heat for the night and not leave it till everything has chilled! Likewise with the watering; there is plenty to debate about watering early or late in the day. Some will hold it is better to water in the morning so the plants can draw on it all day, others that it is better not to chill them then but to give it when they are warmed up in the afternoon. I don't reckon it really matters as long as they always have moist compost but never stand in water for long! It is not a good idea to water towards night though unless the plants are really desperate for it, if you must water late in the day do it carefully with warm water and don't splash it about or on the foliage.
As to how not to water! We mostly splash it in the top of the pot. This can have the effect of wetting the top layer and around the stem neck and giving the appearance of moist compost and all is well whilst underneath the roots are dry. This is ok for only the more robust plants. If our plants are also in saucers the bottom ends of their roots may drown during the unhappy hours after watering whether or not the intervening compost has actually ever been wetted by the water passing through. Yet I reckon on observation this is far too often the commonest method employed.
Although more tedious it is reckoned more effective to plunge the pots a third to half their depth in a bowl of water but only until the compost is moistened and then to immediately and thoroughly drain. Over-done this may water log the compost, and there is some leaching away of nutrients though these can be replaced. But because the rootball is wetted from underneath then the neck where the stem joins the roots is kept dry which is exceedingly important for such as cucumbers and melons. Multicelled polystyrene trays are the devil to water from above but floated on (preferably warmed) water they soon take it up and can be removed and drained. Immersion is also the safest method for rewetting pots of fine seed or tiny seedlings as it does not puddle or pan down the surface. (It is also about the only sensible way of wetting dried out rootballs -add a hint of soap and use warm water to resuscitate them more quickly.)
However in this pragmatic world most of us are likely to prefer to carry on splashing water from a can. Please reserve this usual watering in the top of the pot for the more robust subjects and let me put in a plea for watering from the bottom for all those more sensitive specimens we are trying to grow on such as tender crops. Rather than plunge and drain there is another way. Stand these pots in saucers, preferably over size saucers, then these can be filled with water and the plants allowed to take up what they want. Done but once or twice daily and just left this will risk water logging or/and drought. However by observation it will be noticed that such as tomatoes in full growth quickly take up all or say half the water in their saucer by the time you have finished watering all the others. Thus if on the first pass all the saucers are filled then make a second pass a bit later where only those already emptying are now refilled. Come back a little later again to tip out ALL the saucers EXCEPT those where all or most of the water has been taken up again -these alone can be topped up and fairly safely left full as their rate of consumption is so high.
What about the water itself? Well in many trials there seemed little advantage to using warm rather than cool water -but freezing cold water can hurt tender seedlings. I still prefer to take the chill off water by leaving it in the cans in the greenhouse to warm up before hand or by using it from a small butt kept in there. More important though is to use clean tap water for seedlings until they are growing robustly although later they are usually happier with rain water. This avoids infecting them with damping off moulds whilst small but tap water is not good in the long run for most plants as it contains too many salts. However if you have had a lot of trouble in the past with diseases, especially tomato and cucumber viruses, then it may be better to avoid water from rain water butts as these can harbour infections.
(One important if digressionary point- don't forget that tobacco mosaic virus is very easily transferred to tomatoes. Be very careful never to handle tobacco before tending to your tomatoes or let any unwashed smoker anywhere near them. It may seem minor but this has been shown to be a major cause of problems.)
When the close of the year comes back our watering has to be carefully reduced from the peak of high summer. Again we need to notice the daily changes as under cover on a sunny day, even in mid winter, then some plants can still grow strongly and use up all their water whereas on the next chilly cloudy day they may use next to none. In general once the days shorten again it is safer to under-water than over-water and especially with the more difficult subjects such as over-wintering pepper plants. Likewise although ventilation can be reduced it will still benefis the plants if it can be given whenever possible.