According to Wikipedia:
The Köppen definition of this climate [humid sub-tropical] is for the coldest month's mean temperature to be between −3degC and 18degC, and the warmest month to be above 22degC…… It is either accompanied with a dry winter (cwa) — or has no distinguished dry season (cfa).
Significant amounts of precipitation occur in all seasons in most areas, and though in regions bordering on semi-arid climates (usually at the western margins), irregular droughts can be common and catastrophic to agriculture. Winter rainfall (and sometimes snowfall) is associated with large storms that the westerlies steer from west to east. Most summer rainfall occurs during thunderstorms and an occasional tropical storm, hurricane or cyclone.
|Climate map from here|
One of the challenges of growing food in our part of the sub-tropics is the temperature extremes. You need to plan for both a hot and humid summer and a cold winter. In the summer months, shade and moisture retention are essential. Shade can be achieved both with permanent structures, such as shade cloth or large trees, or more temporary methods, like growing climbing plants up a northern trellis to shade the garden. I have shade cloth covering the entire garden, which is probably more than is really necessary and can make my garden too shady in winter. I also use climbing plants like beans and tromboncino to provide extra summer shade. Water retention is improved by building soil organic matter using compost and mulch. Mulch on top of the soil also helps to prevent evaporation. I water the garden every few days with our grey water, but nothing compares to a good fall of rain from a summer storm.
In winter we get frost. In total we only actually have about 20 frost days per year, but you only need one heavy frost to kill anything and everything that is frost tender. The shade cloth helps to some extent, and I have a small greenhouse to protect tree sapplings that I've started, but my best solution to frost is to just grow frost-hardy plants over winter. Fortunately this includes plants that I can't grow in summer at all, due to bugs, this includes carrots, broccoli and cabbage. They all do much better in winter in the sub-tropics.
Crop rotation is a another challenge in a sub-tropical garden where you can grow something all year round. There is little opportunity for cover crops or a winter fallow, unless one garden bed is to be sacrificed for an entire season, and I don't have the space for that! This just adds a little extra complexity to garden planning, and I at least try to move the tomatoes and broad beans around each year.
The change of season can also be tricky. I still have cherry tomatoes in the garden in June, which is officially winter, but it hasn't got cold enough yet to kill them. Meanwhile, I'm trying to make room for winter crops, which will have a short season as it is, because I can start summer again in September (or earlier in my greenhouse)! The solution is a bigger garden I think!
Veges that grow well in a sub-tropical winter season (with frost)
- brassicas - all asian greens, broccoli, cabbage
- root veges - turnip, swede, carrots, radish
- onions and leeks
- broad beans
- curcubits, but watch out for powdering mildew from the humidity - tromboncino, squash, pumpkin, melons
- capsicum and chillies
- ginger and tumeric
- arrowroot and comfrey
- peas seems to be best in the in-between season
- tomatoes sometimes do better after the summer bugs die off