I have had no luck at all finding you-pick sources for organic food. If you know of any, let me know though the contact page.
Kroes Christmas Tree Farm is 20 minutes from Stratford, open weekends right up until Christmas. At 1903 Perth Line 43 in New Hamburg, ON N3A 3Z5 http://kroeschristmastrees.ca/
Thomas Brothers is about 50 minutes from Stratford, but it’s the only place I know of in these parts where you can come back with a carload of good food very cheaply with just a few hour’s effort. I’ve picked broad beans, yellow and green bush beans, tomatoes (roma and table tomatoes), cucumbers, dill, all kinds of squash, peppers, and hot peppers. Kids love it there, and they generally work pretty hard to help out. It’s a great way to teach them where their food comes from.
Their farm store is at 5856 Colonel Talbot Rd., London. The fields are a mile or so to the east of the store; look on the right hand side for the U Pick sign. https://www.facebook.com/ThomasBrosFarmMarket
Heeman’s (Nissouri Road, near London) is about half an hour from Stratford, and they are becoming famous for their strawberries. You can pick June/July, and it’s a significant savings. https://heeman.ca/heemans-strawberries/pick-your-own/
People say Millar’s Berry Farms have good strawberries and raspberries, but sometimes expensive, and you have to pay $5 per head just to get into the fields (but this is applied to your purchase). I haven’t gone, because it’s on the other side of London, a bit far for me. https://www.facebook.com/pg/millarberryfarms/reviews/?ref=page_internal
Apples, Cherries, Peaches
Appleland is about 40 minutes from Stratford, near London. I think some people will find it a lot of fun, if you like theme park experiences. They have a train ride, a corn maze, things like that. They also have a nice farm store with lots of tourist goodies, and a variety of you-pick apples. However, it does cost 6 dollars per person to get in, and $7 on the weekends. As far as I can see, this is not deducted from your purchase.
Farther afield is Juicy Fruit Orchards, way over in Thedford. They have many varieties of apples, and some cherries, and they’re open May 24 – Oct 31. They also sell lots of garden vegetables, so it might be a fun family trip some weekend.