After week 2 my double IPA is still fermenting and has a healthy yeast cap of about 1 inch. Should I leave it be or stir it into the beer?
The gravity is 1.019, near to what I'm expecting the final to be (1.010).
It smells so good. I will, but can't wait til it's finished. :)
Btw I used Safale US-05, extra pale DME, brewing sugar, citra and tap water.
Opening up the fermenter and stirring doesn't sound like a good idea, but you can definitely shake it while closed. That should wake the yeast up a bit. But keep in mind that when you do aerate the beer, it gets cloudy, so you will have to wait for it to settle anyway. Personally, I've never had any fermentation issues, but maybe it's because I live in a pretty warm place.
Absolutely do NOT stir it in. You'll re-oxygenate the wort and get weird flavours going on and there's no benefit anyway. it's top fermenting yeast so it's supposed to be on top and will sink at the end. To be honest though I'm surprised it's taking so long even with dry yeast. I did a Mangrove Jack's Mr Simcoe recipe pack in my Grainfather and that dry yeast took about 5 days to start bubbling and finished about three weeks after that for a single so maybe it's just a dry yeast thing. For my own double IPA I use liquid WYeast 1056 with a starter which starts bubbling after about 5 hours and finished ferm in 5 days every time.
2 schools of thought, one is to sprinkle over the top of the wort. The other is to stir then add the yeast. For me I like stirring because it get's air to wort and the Suger to the yeast quicker.
Two weeks doesn't seem like an inordinately long time for a true double IPA. Also depending on the OG 1.019 might be the bottom. As this was an extract beer I'd be surprised to see a double IPA go to 1.010; unless this is something you've brewed before and gotten to that point.
I'd say warm it up to 70-72F and give it another week.
Stirring, IMO, is a bad idea as it will introduce some oxygen. While that might be great for the yeast it might be bad for the beer.
If you don't mind sacrificing some beer for you own education. Pull off on gallon and split it into two sanitized containers. To one go ahead and swirl it, get some oxygen into it. Let it ferment another week and see if it goes lower than 1.019. That will show you the effect of more O2 in the beer. In the second half gallon, add a half packet of dried yeast and swirl the thing every couple days for a week keeping it as warm as possible. This is known as a forced fermentation test.
Next week check the gravity of the two half gallon samples and the main batch. If the sample with the extra yeast has fermented down then you know there is still fermentables in there that CAN be fermented out. Hopefully the main batch will have moved along some too.
Stirring won't hurt it. US-05 is not generally problematically flocculent so you shouldn't have to though. If there is still a cap of kraeusen on your beer, it's still fermenting so I'd probably leave it alone myself.
Two weeks is a long time for fermentation to finish out. If you are near the bottom of the yeasts temp range, I'd suggest moving the fermenter to a warmer place if you want to do something to get it to finish out a bit quicker.
Has the yeast settled to the bottom? I'd let that happen rather than stir it in.
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