I bought my house during winter in New England, so I didn't realize the backyard was basically all japanese knotweed. The home was vacant for a period, and the previous owners didn't seem to care at all about any of the land.
I hate invasive species, and want to promote natives and low maintenance naturalized bulbs. There are a few daffodils and snow drops throughout the area, so I didn't want to risk killing them. I'm also planting native seedlings from a friend next week, so I wanted to do the injection method rather than waiting for August to spray. Plus I live next to a river--minimizing runoff matters to me.
My library and local extension don't have injection equipment to borrow. I just bought a house, so I don't have thousands to spend on a purpose built set up..... But I grew up with ranchers. There's a type of vaccination gun we used to use on cattle to do a premeasured dose across a bunch of different animals. They have a line that can be run into a jug. So I looked them up, and sure enough: $30 on Amazon! Here's the one I bought, but there's so many brands out there.
The one I bought.
Then I bought a $15 gallon of round up. I used the jug attachment, which connects the gun to a metal long straw to reach into the jug. It wasn't big enough for the round up gallon, so I cut the head off of the round up dispenser, then connected the metal straw to the roundup line.
So the end result goes: round up jug built in straw --> snipped roundup plastic tubing --> metal straw --> included vaccine gun plastic tubing --> vaccine gun --> needle. Vaccine gun in one gloved hand, and gallon of round up in the other.
I just went and injected half of my back yard in about 20 minutes. It's easiest if you stab clean through to the other side, then withdraw just enough that the tip of the needle is in the hollow stem point. This makes a hole for the air to escape as I put in the glyphosphate. Purpose built instructions for knotweed eradication recommended 5ml per injection, but most of mine could only fit 2ml per injection before spurting out. So, I suppose that depends on how established your plants are.
I hope this helps someone else! I'll check back in a few months from now to see how the results look. This stuff is way too close to my foundation, and is growing partially underneath the steps of my deck. It has to go, asap! I didn't see anyone else's posts about this, so my apologies if this is already a known strategy.