That seems a good idea, since the plants useful for your dragon surely are used to the same climate of it.
You could plante it if
- the soil is the same that you use for your animal
- the manure is not dangerous for your animal
- there is space enough for animal and plants
I can start with saying that spinach, broccoli and Kale (Brassica oleracea Acephala Group, a vegetable similar to cabbage), are Winter plants = You can start planting these in the summer to have a harvest in winter.
Turnip green are winter / spring
The benefits derived from turnip husbandry are of great magnitude; light soils are cultivated with profit and facility; abundance of food is provided for man and beast; the earth is turned to the uses for which it is physically calculated, and by being suitably cleaned with this preparatory crop, a bed is provided for grass seeds, wherein they flourish and prosper with greater vigor than after any other preparation.
Turnip (flower) The leaves of turnips are also eaten as "turnip greens"
The first ploughing is given immediately after harvest, or as soon as the wheat seed is finished, either in length or across the field, as circumstances may seem to require. In this state the ground remains till the oat seed is finished, when a second ploughing is given to it, usually in a contrary direction to the first.
Mustard greens (Brassica juncea) - Mustard plant are any of several plant species in the genera Brassica and Sinapis. Mustard seed is used as a spice. Grinding and mixing the seeds with water, vinegar or other liquids, creates the yellow condiment known as mustard. The seeds can also be pressed to make mustard oil, and the edible leaves can be eaten as mustard greens. Mustard seeds generally take three to ten days to germinate if placed under the proper conditions, which include a cold atmosphere and relatively moist soil. Mature mustard plants grow into shrubs.
Pea sativum is an annual plant, with a life cycle of one year. It is a cool season crop grown in many parts of the world; planting can take place from winter to early summer depending on location.
Each crop has its own rules, but in general they are easy. Once you get started, then you would find yourself well, or you can even ask for advice.
My fear is that the product obtained is not enough for your dragon, that would ultimately devastate your garden. Surely you will integrate the crop with vegetables purchased, unless your soil is so large as to be able to grow food enough.