With a greenhouse we can start producing crops months sooner than outdoors. Most advantageous is bringing in tub grown fruit trees and bushes to force earlier, weather and bird-proof, harvests. Indeed bringing in Peaches, and even more so, Nectarines, in tubs is THE way to prevent Peach Leaf Curl, and ripen perfect fruits, earlier. Apricots are also well worth bringing in. Apples are marginal, Cherries, Pears and Plums are difficult, as are Raspberries.
Gooseberries are easy, earlier and sweeter when brought under cover. Blueberries are well suited, already grown in tubs of acidic compost their main problem are birds so bringing them under cover solves that, and gives earlier crops. Likewise Strawberries, best grown one plant per bucket sized tub these are easily moved, and by bringing in another tub each week you ensure a continual supply of fruit for a month or more well before outdoor ones start to ripen.
Although their yields may not be heavy growing New Potatoes in tubs under cover will have you eating yours at Easter when others are only planting theirs. Plant one set (small seed potato) of an Early (fast) variety, especially Rocket, per tub. Feed and water well once the plants are growing and as with strawberries start a new batch every week or so to get continuity of supply.
Now a frost free greenhouse is all you need for many crops but you need a warm propagator for starting some, especially Sweet Potatoes. These are really worth growing but only if you can start soon enough as yields are much more rewarding with longer seasons. Pot a sweet potato and keep warm, shoots will appear, once these are finger length or so de-pot to detach these. Pot each up individually to grow on in large tubs (up canes) and you’ll get fantastic crops.
You also need a warm propagator for starting Cucumbers, but amazingly sow a modern all female small fruited variety now and you’ll be eating cucumber sandwiches along with your new potatoes! By the end of January you can also sow first batches of indoor Tomatoes, Chilli & Sweet Peppers and Aubergines.
Fortunately you’ll need no heat for sowing Onions and Leeks in trays or cells. The sooner you sow these the better, sow thinly and plant out later when your garden soil has become friable.