Thickening berry pies -- including strawberry pies -- is a bit more difficult than thickening orchard fruit pies. Flour can thicken them, but it's not your only option. Berries exude a lot of juice, so let them sit in sugar for 30 minutes and then drain off the excess liquid. Whether you use flour, cornstarch or some other thickener in your strawberry pie, bake the pie at an oven temperature of 350 degrees Fahrenheit, until the filling is bubbly and thick.
Flour thickens strawberry pies reasonably well, although you'll need more flour when thickening strawberry -- and other berry pies -- than you would if you were making, say, an apple pie. Use more flour to thicken pies made from frozen berries, versus fresh. You'll also use more flour if the berries are very ripe and juicy. Pies thickened with flour may continue to thicken after you remove them from the oven. Like cornstarch, flour breaks down and becomes watery if you freeze a strawberry pie thickened with it.
The main disadvantage to using flour as a pie thickener is its appearance. Flour tends to make pie fillings cloudy, which isn't obviously apparent in pies made with orchard fruits, but does mar the ruby-red brightness of a strawberry pie. If you use too much flour, the texture may be gummy and thick. Too little flour, and the filling will slump or run.
Another potential problem is the taste flour imparts to strawberry filling. Flour gives a slight grainy taste to pies thickened with it. It's most noticeable in delicate berry pies. You might not notice the taste, but if it bothers you, switch to a different thickener.
Flour is economical, widely available and simple to use as a pie thickener. You can use it to thicken strawberry pies, and if you're happy with the results, continue to use it. However, cornstarch, tapioca starch and Clear Jel all provide superior results when it comes to filling strawberry pies. They all thicken reliably, have a neutral flavor and in general, give a clear, translucent finish to strawberry pies. Cornstarch may occasionally become cloudy if the strawberries are very acidic. Pies thickened with Clear Jel can be frozen with no ill effects. Sprinkle a few graham cracker crumbs in the bottom of the piecrust to sop up any extra juices.