does a bubbling airlock mean I didn't underpitch?

So I'll be the first to admit I'm still pretty new at this, but I had success with my first AG batch so I decided to step it up and brew an imperial AG last night.
I ended up with about 5.5 gallons of 1.080 wort but like a dummy I didn't realize that worts with that high of an OG need a starter or a double pitch to effectively kickstart fermentation. Regardless, I pitched my vial of White Labs London Ale yeast and I had active fermentation (white krausen, bubbles out of the airlock) within 12 hours or so. About 20 hours later right now and the krausen is 2 inches thick and the airlock it bubbling like mad. Does this mean I didn't underpitch, or will I not really be able to tell until I taste it? I have another pack of yeast I can throw in if you think it will help, but it's witbier yeast so I'm hesitant. Although london ale and witbier might be an interesting combo....

Thanks in advance!

Well, you definitely underpitched but that's OK. One vial is still going to make beer. Look into making starters so you can pitch more yeast from the same vial and step up the cell count a bit.

Don't keep checking on the krausen, it won't miss you if you don't check in for a while. ;-) You really won't be able to tell until you taste it--but I think you'll be fine.

Hey Mike, thanks for the quick response!

What's the downside to underpitching? Would it be worth it to add the witbier yeast to try and increase cell count or do you think my attenuation will still be fine? Perhaps just wait longer in primary before racking to secondary?

Yes, you under-pitched.

No, adding more yeast will not help. Well not help any of the affects of under-pitching.

You may not notice anything, but under-pitching can result in stalled fermentation (high FG) and high esters and fusel alcohols.

Just wait and see what happens.

You didn't mention what the beer actually was... some beers can benefit from intentional underpitching, such as belgians, where the esters are a large part of the flavor profile. Other beers, such as an IPA, should be overpitched IMO. I don't want my IPAs tasting like apricots...

It's a holiday spiced Pumpkin ale, very similar to the recipe below, except that I traded out a pound of the American Pale malt for a pound of Biscuit malt. Honestly, some esters might not be a terrible thing for this beer if they lend a bit of a fruity flavor.

https://www.homebrewtalk.com/f76/imperial-pumpkin-pie-ale-200122/

Like everyone said, maybe some unusual flavor profiles and a late stall but nothing crazy. As long as the yeast was kept healthy and cool and was within it's use by date. IMHO a witbier yeast would make a pumpkin ale taste odd, but stranger brews have been made.

I would save the witbier yeast and not add that; the other yeast is running things now.. Keep the fermenter in the low 70's. Overall attenuation is a function of other things--like mash temp or aeration for example. So it's not just affected by cell counts. Pretty sure you are going to have beer in a few weeks.

Right on, thanks! The yeast was very fresh, my LHBS is absolute garbage, but about an hour away there's an amazing brew store that takes this very seriously and has amazing products on hand and that's where I go for all my grains, hops, yeast etc.

I mashed at around 155 for 90 minutes. Had some fluctuations of course because it's really hard to maintain an exact temp in the entire mash tun for an hour and a half but I think I did pretty well. Wort was very well aerated, I find that's an extremely easy step that's really important so I always pour my wort back and forth in my two brewpots 5-6 times before it goes in the carboy.

Thanks for all the help again. If anyone thinks it's worth it I could hit the brew store tomorrow and pitch another vial, but I guess at this point the cell count in there is already high enough that whatever damage will be done has already occurred.

So I just woke up to such a vigorous fermentation that the airlock had blown completely off and I literally had bits of hops and krausen on the ceiling. So yeah, I guess that's going well

I replaced the airlock with a blowoff hose until the fermentation settles down a bit. Anyone think I have to worry about infection now? It was bubbling so hard there was a serious pile of foam and debris all over the place so I have a hard time picturing anything getting IN throughout all that, but I guess it's worth asking.

I doubt you have an infection. My buddies brew actually blew the lid off his bucket, nevermind the airlock. His beer turned out fine.

I guess there are 2 takeaways from this thread:
1: Use starters.
2: Imperial (insert beer here) = blowoff tube!

Look at it this way; at least you got one more look at the krausen. ;-) I've had a few like this, since the stuff was gushing out--not too much could have gotten in. An active krausen is a pretty inhospitable place for anything other than the yeast.