Welcome to Workshop @Jeannot. It's great to have you join us.
Please note I've moved your post to the gardening board and added a more descriptive title so more community members can see it. I'm sure our members will have plenty of helpful suggestions for you about getting rid of weeds in your lawn.
Onion weed isn't a big problem in most lawns. If its popping up it's just a sign that your lawn needs some love. If you try pulling it up you'll probably just make the problem worse as you'll spread the bulbs. Best way is to just ensure your lawn is healthy and growing fast. It will soon outgrow and dominate the onion weed. You could try things like boiling water or Glysophate but they will obviously also kill your grass.
The Buffalo lawn was turfed about 20 years ago & did well, getting thicker/running in Summer as its getting the sun & thinner in winter as it is mostly in the shade with watering system adjusted accordingly & very little traffic on the loan, for some unknow reason the last 2-3 years, every other grass or weed type seems to be getting on top of the buffalo grass ? perhaps too old or needing complete re-turf ?your advice appreciated
Buffalo grass from my recollection is a very thick lawn that tends to thatch over time, so my guess would be to perhaps scarify it, aerate, & feed before the hot season(?).
*Late Correction, DO NOT scarify, please refer to @greygardener's superb linked article.
Regular feeding is important. This might help - http://buffalolawncare.com.au/buffalo-grass-lawn-care/147-buffalo-lawn-care-tips.html
Excellent & timely article @greygardener, I had no personal experience of Buffalo, & through ignorance grouped it with couches, so wrongly recommended scarifying.
I was hoping when I submitted, that you'd confirm or correct me, so thanks for clearing it up.
Thanks to both of you , it does look like that is what happened, thatch build up, I will follow advise from the link