Autumn is fine for planting a lemon
tree in a warm climate. It won't visibly grow much thru winter but it will be well settled come
spring and will grow well. The most important thing though is to ensure the soil is right for citrus. Good drainage is essential, plenty of organic matter, no competition from
weeds or grasses so mulching is important. If you have made these preparations then go for it. Otherwise, i'd suggest you spend the rest of the cool season preparing the soil and
plant in
spring.