Posted: 23/03/2015 at 22:26
Marigolds don't actually deter greenfly/blackfly in my opinion, but they do attract hoverflies which feed on them. Usual result is less if not no pests. I did try it for myself planting them together in the same pot, the results were fewer tomatoes, so now I put them near the tomatoes again fewer pests and more tomatoes.
There was a recent test in GW mag about feeding and watering toms and watering every 3 days seemed to produce the most toms with feeding once a week. I don't think it makes much difference when coming to bags or pots although as pansy says pots are easier to move and tomatoes aren't deep rooted plants anyway they spread to the space available in bags, and depth of pot ,I find, doesn't seem to make a difference just the width at least 8 inches. As for feed I buy Wilko own brand, they are all made by the same people it's the cost that's the difference and personal preference.
Best of luck and feel free to ask as many questions as you like, the only stupid question is the one you don't ask.