I used to think it was strictly the size of the fruit on a blackberry that told you whether or not it was ripe. Later I changed my mind and believed that it was the number of fruit (the overall size of the berry) that was the better indicator. Now, I am certain that while both of these things are good hints of ripeness, the true litmus test is how easily it comes off the vine. Too easy and you have an overripe berry that "tastes like water" according to Sahsez. Too difficult (you have to tug on it) and you have an overly tart berry. Just easy enough (the slightest tug does it, but it doesn't fall off when you merely move the vine) means you've got a nice, sweet, flavourful, juicy berry.
This year, despite having made jam pretty recently with a freezer find of 2kg of blackberries, Sahsez and I went picking around our neighbourhood and made 1kg's worth of jam. We were in Europe during the season last year and despite my neighbour's helpful suggestion that we just ask Abu to pick for us and freeze them and I could make the jam once we got back from the vacation (HA!) we didn't get to do blackberry jam in 2005.
(I use Nigella Lawson's recipe from How to Be a Domestic Goddess and it goes like this: take 1kg blackberries, 1kg sugar, and a lemon's worth of juice, bring it to a boil, let it boil to a setting point, then toss in sterilized jars. People, it's so easy and people, I can't tell you how much people love it. My lovely friend Ms. T says it is "awesome". And she's from Saskatchewan so I think she knows what she's talking about.)
For the first time, I got smart about my hands and brought gardening gloves to protect them during the picking. This helped a great deal as I could reach for a high branch, hold it with the gloved hand, and get all the berries with my bare hand. Wish I could say I was smart about the rest of my picking outfit as well. But I wasn't. It might actually be smarter to wear bare legs than it is to wear luon pants, which actually attract the prickles to your skin. Sahsez and I were pretty aggressive, crouching down low and getting a lot of scratches to reach the ripest blackberries.
Sahsez was very creative in fashioning picking tools out of broken tree branches and even used one to smoosh down a whole bunch of prickly branches to allow us access to the juicy fruit behind them. She was a real trooper and way outlasted me in picking energy. I was begging to go home while she was still finding new blackberry paydirt.
I learned another lesson about blackberry picking this year: when you're walking along and picking and eating as you go, make sure you save at least one reliably ripe berry from each bush to eat last. Otherwise, you may find that you have inadvertently eaten a tart one last and that tart taste will linger on your tongue until you come across another bush, which could be a kilometer away. I learned this during a crack-of-dawn walk with M'hijo on Saltspring a few weeks ago, during which I not only picked and ate my fill, but brought home 1/2 pound for Sahsez' breakfast. She gobbled them down greedily, in handfuls, before her dad got up. This is another way to go if you don't feel like making jam.

