Alan G is away so Barb C says: Suggestions for September: I've gone to Alan G's 'go to' Yates Guide (which is up in the middle hot house). This guide, for a cooler climate, says we can plant the following in September
Climbing peas. Last month to put in peas except for sugar peas
What To Plant In September by John Upsher August provided glimpses but September is the true start of spring with warmer days and nights with no chance of frosts. Day temperatures will not be reliably warm enough for the crops that need those higher temperatures. If you put in some of these this month they will sit there and may well deteriorate waiting for the heat they need. Better to wait until October or November to put in melons, pumpkins, basil, eggplant, capsicum, peppers, and sweet potato. However September is warm enough for you to put in many of those crops that you will want to be harvesting before Christmas.
This is the last month to put in peas as they get powdery mildew in the warmer days of November and December. Sugar peas on the other hand are OK for another month and go well with late spring salads. Later in the month, dwarf (french) beans can go in.
Greens are great to go in NOW, either as seeds or seedlings. Lettuce, spinach, rocket, Chinese greens, spring onions. Give the soil a nitrogenous fertilizer, like chicken manure for these. Similarly the cabbage family may not do so well through the heat of summer but you will get good crops before Christmas. Consider broccoli, mini-cauliflower, cabbages and the kales.
Root crops are best sown as seed rather than seedlings from a punnet because in the punnet the roots become distorted and planting out can cause damage to them. We have beautiful soil so get the best root crops you can by sowing seed. You can put in beetroot, carrots, and radish from now until November. After that, they suffer in the summer heat at a time they should not be stressed. Potatoes planted now can give you earlies for Christmas. Planted in October they will be your main-crop varieties to be lifted in March and April
You can start planting your tomatoes now but choose your varieties. Pick not-tall (that do not need staking or pruning), EARLY varieties like KY1 or Siberian. You will get your best tomatoes from these early plantings through January. It is best then to put a later type in about November that will crop through February to April. The same with capsicum, eggplant, cucumber, sweet potato and zucchini, but there is not a lot gained by putting these in the ground before October.
First sowings of sweet corn can go in now. These can grow to quite large plants so put the seed about 35cm apart where they will not shade other crops. Only grow as many as you need for 3 weeks. Leave space for more to go in a month later.