Well, all our avocado trees - the main types being Bacon, Fuerte, Hass (with the odd Shepard and Zutano tree thrown in for pollination) produce fruit over the summer which matures ready for harvest in winter. This means that harvest time in the northern hemisphere will be anytime between October to March and we will decide when to pick depending on the fat oil content of the fruit. Some types are ready earlier than others - for example, Bacon is picked before Fuerte, which comes before Hass. To find out the oil content, we simply take sample fruits down to the co-operative, and they test it for us. If the avocados haven't reached optimum fat content, then we simply wait to pick them. Anything picked outside of this time frame is a summer avocado.
And there's the rub - there are two types of summer avocado: one is delicious and the other is not.
A summer avocado is an avocado that was "overlooked" when the pickers picked the orchard and left a few on the tree - way, way up on the top branches or hidden beneath the leaves. Avocados can "hang" on a tree for many, many months. They don't ripen until they are picked or when they fall off the tree. So finding a few avocados (especially the Hass type) left on a tree is a special treat, seeing as in the summer there are no avos to be had locally. They just keep adding oil and get yummier and yummier. (But if they are left too long - they can go rancid... so its a delicate balance here...).
And then there are those avocados that were picked TOO early, perhaps one or two months before their optimum date and oil content. Why would anyone do this? Well, perhaps the farmer was tempted by the high price he would get, or maybe he feared bad weather... But the fact is, whatever the motives, there are avos that are picked too early on in the season. Which means they are not ready to be eaten and never will be, because its oil content is too low. The flesh will never really ripen. It will slowly shrink along it's ridges and just rot. When avocados are picked too early, they just get wrinkly (usually from the top down) and stay rubbery.
And last but not least, you can also get unscrupulous farmers who pump their avo plantation so full of water and fertilizer that the fruit seems to bloat and seems to be ready before time. Again, when you open the avo, what you will find is stringy, watery flesh that tastes of nothing.
Remember, a Hass avo will turn blackish green when it is ripe and the Fuerte and Bacon will remain green but will feel soft to the touch when ripe. Good luck finding the perfect fruit!