There is probably no more widely grown crop than tomatoes. Almost every kitchen garden will have some growing indoors or out and I doubt there is any plant more often tried by the novice. There are hundreds of varieties of very differing value, and success with the outdoor ones is almost entirely dependent on the year rather than the gardener's skill.
Indeed the last few years have been very trying for outdoor tomatoes and many gardeners have been depressed at their miserable performance all summer to then despair when the plants succumbed to potato blight just as they looked like eventually giving some ripe fruit.
Unfortunately most advice on growing tomatoes is given by one of two camps; the grow-it-for-show'ers with their beautiful billiard balls of Goldstar (almost the most pathetic eating you can find) and the commercial boys who crop equally tasteless red balls for fifty weeks a year. The first lot tell us how to produce a few tasteless inedible prize winning fruits and the second how to produce the maximum weight of equally tasteless inedible but long keeping fruits per square foot of growing space. Neither are much concerned with producing the tastiest fruits by the laziest method which is what I expect most of us aim for. And both are not really concerned and so have little practical advice to give for outdoor crops.
So first of all you have to ask yourself the simple question "Why am I growing tomatoes?" If it is for fun fine, if it is for taste then do not slavishly follow the conventional advice and if it is for economy then either go under glass or plastic or grow something more reliable! Of course if you have a warm sheltered garden on a south facing slope with a warm border in front of a hot brick wall you will not know what I'm going on about. However in any garden that is less perfectly situated the outdoor tomato crop does require some skill and luck to give good results and especially during the current run of summers.
In any case if you want an early crop go under cover instead. Do not try for an early outdoor crop as most of the difficulties become lessened if the plants can avoid the coldest most risky period. Transplanting out outdoor tomatoes is far more successful if not done by mid-May as recommended but left till several more weeks have elapsed. All chance of a late frost is then over and the soil has actually warmed up. I have become fed up with putting out beautifully grown plants to see them wither and fade in the cool damp weather of the last few early summers.
Thus it makes no sense to start off the outdoor plants early in March as then they will just get pot bound unless you can keep moving them up a size. Start off your seedlings in April and do sow them individually in small cells or pots of good compost. (I use my own, in trials it's proved as good or better than any bought in ones) Overcrowding as seedlings even if only for a few days can halve yields so do sow and grow on singly. And do not grow, or buy, too many plants, in fact you can grow fewer than you need as each plant of a variety can give up to a dozen more by propagating the sideshoots normally removed anyway.
If the sideshoots are detached when a few inches long they can be potted up and rooted quickly especially if some bottom heat is given. These plants are the same biological age as their parent so flower and fruit much quicker than more seedlings would, and they do so from lower down which makes for usefully stocky plants. Do not cut off the leading shoot as this sets back the cropping of the main plant.
It is also not necessary to remove all the sideshoots as recommended. I find that if I leave a strong growing sideshoot from the base then I can grow plants with two stems instead of one. This saves on the number of plants to fill a row and gives about as good a yield as two single stemmed plants. Three stems also work but do give less toatal crop per yard. However the great advantage of twin stems is there are half as many plants to plant and water and there are far fewer sideshoots to remove as often as the twin leaders take up more of the plant's energy.
You can buy in plants if you can get them of suitable varieties, however the range of plants on offer is nowhere like that of those you can grow from seed. It is certainly a good idea to try several and find which suits your local conditions and your taste. Some are so good though that they are sold almost everywhere; there are few tomatoes as popular in taste trials as Gardeners Delight, this is good indoors or out and always tastes best from outdoors. There are many reputedly reliable outdoor tomatoes but they are all now suffering from what is fast becoming a major problem.
Potato blight also attacks tomatoes, and has been doing so more, and earlier, over the last few summers. There are a few blight resistant potatoes and the early sorts can be grown to avoid it but for tomatoes neither option is available. The early potatoes are already dying back in July when the blight most often arrives, and attacks this early for tomatoes are invariably fatal and well before much fruit can be cropped. Some small fruited and most plum tomatoes seem to be the longest survivors but it is sods law that the best flavoured such as Pink Brandywine seem most susceptible.
Indoor crops are not often affected or are only late in the season. An alternative I have come up with is for the outdoor crop to be kept dry with a plastic flysheet. This gives sufficient protection to the plants as it allows almost full air ventilation while the rain as such is kept off. With the foliage kept dry the blight is kept off too unless very humid damp weather persists when nothing helps.
I was already using a stout pole atop two strong posts to support a row of tomatoes as this is simpler and stronger than a load of separate canes. Initially I was just after the frost protection when I put a clear plastic sheet over the row forming a tent with open ends. My idea was to remove the cover once the plants had established and the hot dry weather had started but the protection from wet and blight was so obvious I left it on but opened it out flatter like a flysheet.
The plants got enough light and air to really taste whereas if they had been more enclosed they would have been as insipid as other indoor grown crops. As the method was so useful I wanted to improve the light without losing the shelter. So the next year I made the support run East West and put a clear plastic sheet roof over that was lop sided with only one wall at the back and no sides. At the front this flysheet was pulled out and down by almost a foot so light and air could reach the plants and at the back it extended back and down almost to the ground. The plants were put in at the back so their roots received the drips from the roof and were tied on strings to reach forward up under the roof to the crossbar so they sloped forward at about 45° allowing their trusses to hang underneath. I recommend you do the same.
One other technique I have used to improve the outdoor crop is leading shoot replacement. What I noticed was that the plants put out early often stalled, or rather the main stem stalled for several weeks, until the soil warmed up. By then the main stem had hardened and although growth started again at it's apex the shoots from low down often looked much fatter and more vigorously healthy. So I tried leaving on one or two of these as I recommended above for multi-stem plants. Because the old stem had hardened the new one proved so much better now I suggest that when the plants are put out that their existing stems are worked over horizontally to be cropped of any set bunches and then removed whilst the strongest basal shoot, or two, is retained and tied up to the supports to carry the bulk of the crop.
And of course another factor is feeding. Almost all the books recommend feeding tomatoes gross amounts. If you want gigantic yields of tasteless green balls then feed freely but beware that every time you feed you set back the crops ripening! Do not starve your plants but if they are hungrier not fatter they will make tastier and earlier crops. I enrich their site beforehand with compost and wood ash but I only feed plants in pots never those in the ground indoors or out. And even those in pots are underfed not overfed -I want tomatoes not masses of luxuriant foliage!
(Incidentally the plants in pots are trials for feeding regimes. I really cannot be bothered to try and crop tomato in pots or bags as the restricted root run means they never taste as good and they need too much assiduous watering to avoid blossom end rot. Which seems to only be a problem when toms are confined in such ways.)
Growing under cover is by far the best way of improving the reliability of the crop even though they never taste quite as good as those grown outdoors. At least you are guaranteed them under cover. And so many more varieties are possible under cover such as the excellent large fruited Dombito. There they may avoid blight but moulds and mildews of all sorts can still be a problem. Some of the newer greenhouse sorts such as Aromata and Moravi are resistant to most of these and crop for months longer than older and gourmet varieties. Mind you some new ones such as the small fruited Sakura have excellent flavour.
Being organic I avoid chemical pest control but have rarely seen many pest problems probably as I always have French marigolds in abundance. The only time I ever saw a whitefly in my greenhouse was the year my marigolds were lost and not replaced! I no longer use my greenhouse as my polytunnel is so much bigger and more convenient. However I reckon the tomatoes do not do as well under plastic as they do under glass (heat and weather being assumed the same). If you have the choice I think they prefer the brighter drier conditions under glass and taste better grown there, but still nowhere near as good as those from outdoors!