For cropping indoors, September and the first half of October is time to sow seed such as chervil, lettuce, spinach, coriander, land cress, winter purslane and oriental leaves such as mustards of many types, pak choi, leaf radish and salad rocket. To have plants which crop together for much of the winter period – say November to April – sowing dates are best staggered and I have written the vegetables above in order of sowing, so chervil is best sown early September and oriental leaves in late September or early October.
Sowing in November to January is possible indoors but germination and early growth will be extremely slow, so first harvests may be delayed until March. Only when plants have a root system, can they make the most of any growing opportunities to develop new leaves of worthwhile size.
I raise plants in modules and plant them out as soon as tomatoes, cucumbers, aubergines, basil etc finish cropping in early autumn. I aim to finish planting by the end of October so that there is still a little growing warmth and daylight for plants to establish before winter sets in. Young plants resist cold and damp better than large ones, but only as long as they have got some roots down and are comfortably settled in. A few outer leaves can be picked off in November and December, but pre-Christmas is essentially a period of establishment for the main productive months of late February, March and April, when increasing light levels bring about an explosion of new growth.
Yes but most leaves in winter are created on older plants with established root systems. New seedlings need time and energy (the warmth you give them included) to reach a stage where they can offer worthwhile harvests. So it is best to have sown in September, then pick leaves carefully off these plants all winter. Any extra warmth for existing plants will certainly result in more leaves, but I do emphasise that in mild weather there should be a reasonable number of small new leaves on indoor, unheated plants.