Hi! I assume that you've been feeding more solid foods lately as your DD gets older? Are you also using formula, or are you breastfeeding?
If you are breastfeeding, a simple solution is to cut back a bit on the solid food, and breastfeed more. Breastmilk is a laxative. If you are using formula, the iron in the formula could be the issue. You may want to check with your pediatrician and make sure you aren't giving too much formula to your DD--I know there is an upper limit somewhere that most peds reccommend, but I'm not sure what it is--I never used formula.
As far as foods...most fruits and veggies help with passing stools. Canteloupe seems to help soften things up, as does prunes. Definitely also make sure your DD is drinking enough water in this hot weather.
I will also note...a couple of people mentioned making up your own food for your daughter, while others said things like "she's too young for 'real' food." Ummm...what ever did we do before Gerber came along? ;-) None of my kids really liked babyfood. My first daughter absolutely refused to eat baby cereal (we ground up cheerios and she would eat a bit of that), and ate very little of the other jarred foods. Once I accepted with DD#2 that jarred foods didn't need to be the main solid food supple, but rather just a conveniance when I didn't have time to find something "real" that was appropriate for her (and then her 2 brothers that followed), things went a lot better. She ate beef barbeque when she was just about 8 months old! She was trying to get my sandwich, and I thought that I'd just give her some to get her to quit bugging me...well that was the end of bland baby food for her! Go slow of course with seasoned foods...but in many cultures they give seasoned soft foods even to small babies. It may help your child to be a more adventurous eater as she gets older.
Good luck!