The banana/apple trick works if the tomatoes are in a bag but I'd say it would be doubtful out in the open. Don't worry about sunscald until late June.
Picking a green tomato will always lead to a hard core. Sometimes, and this is what commercial growers who sell to grocery stores do, is pick the tomatoes when in the white stage and then they gas them to make the red. This is why they resemble hockey pucks and taste like cardboard. If you wait until they have a very light pink tinge on any part of the tomato it is a sexually mature tomato and will ripen perfectly. I pick ALL of my tomatoes at first blush because of all the mockingbirds, orioles and varmints here. If I didn't they would get them all. Picking a green tomato is only good as fried green tomatoes which I think is a waste of a good tomato. Oh, and please update your profile as to what town you are close to. North Texas is a whole world of difference than South Texas and East Texas is different than both and West Texas is just about impossible. I've lived and gardened in all of them.When I pick a tomato green and let it ripen indoors, usually it will still have a hard core, which is why I just leave it on the plant until it finally matures. I have no animals or birds that come after my tomatoes(I have a fenced in backyard). Don't worry, I'll keep patient, and try to keep my potted tomatoes in the sun as much as possible...if it would stop raining!
Birds never touch my tomatoes. But I have had worm trouble before. Hard cores can also be caused my poor weather to, I think. My German Pinks and Blue Beautys have had that problem before. A few years back I was standing in my kitchen with a knife cutting out the core of each Blue Beauty. It was great though, because I couldn't stop nibbling! Blue Beauty is truly one of my favorites! It may not taste like a red tomato, but it's not sweet, which is not my preference. It has it's own mild, unique flavor. If you haven't tried it, give it a try sometime!
Heirloom tomato varieties often have unappetizing aspects attributed to them. That is why there are so many hybrids, tomato breeders trying to get rid of the things they don't like, trying to come up with a perfect tomato. Weather, I don't believe has anything to do with a hard core.. It's a varietal issue. But I could be wrong on this. I thought mocking birds were all over Texas. If you want some maybe I can scare some your way. For the past 3 seasons I have had trouble with tomato pin worms and haven't found a solution yet except to never stop spraying Bt. If I were still growing commercially I'd be bankrupt but they only destroy a small portion of the tomato so I just cut the rotten part out. And I get the occasional hornworm. I kind of look forward to them as they make great catfish bait.Which is why store-bought tomatoes will NEVER beat a home-grown one!
Mocking birds love to eat tomatoes so watch out for them when your tomatoes get red or purple or any other color. They don't like unripe tomatoes.
