When using the search feature, the results are often obfuscated by answers to the questionnaire questions.

For example, a search for "phalaris" yields tons of questionnaire results to scroll though (many of which are incorrect or sloppy; not that it matters in this case). The first 20 pages only contain 3 proper posts. I haven't counted how many search results pages you have to flip through to discover the "Phalaris - The Way of The Future" thread, the meat of the Phalaris discussion on the Nexus.
For the search term "brachystachys", there is not even one proper post on any of the 40 pages that the search returns. In fact, for a post with "brachystachys" in it to appear in search, it would have to have been made in the last three weeks to beat the constant influx of questionnaire results.

I can limit the search to a specific forum section, but there doesn't seem to be an option to exclude one.

Would it be possible to add the option to exclude the questionnaire results from search and make it the default setting?

Another option would be to use google to search this site.
I tried using your brachystachys example to show how to exclude topics but google seems to inherently remove all the 'New member questionnaire results' anyway
But...
Type your search terms into google, then add site:www.dmt-nexus.me to search only this site.
After that you can use intitle: to search for threads with a word in the title, for instance intitle:brachystachys
You can use -intitle: to remove results with a certain word in the title.
Another thing, less useful on this site, is inurl: and -inurl:

You can put these together in fun ways. This sites search isnt that great for searching with both author and content search criteria. Lets say I vaguely remembered a thread with Desmanthus in the title where nen888 mentioned leptolobus, so I put this in google:
nen888 leptolobus site:www.dmt-nexus.me intitleesmanthus
I get a nice little list of Desmanthus threads that nen888 participated in and that mention leptolobus.

GIYF

You could filter the search by selecting the subforum and not All Forums.
It will demand to make more search, but it will be filtered from questionnaire results.

Thanks guys, google search should do in the most frustrating cases - although I'd prefer to avoid Google due to tracking.

Not sure if this is frowned upon but in theory you could make a dummy new member account and use it only to search since new accounts don't have access to the questionnaire.