Look! Is this Drymaria a half-dead or a half-alive weed?
Does limonene work as an herbicide? Two weeks after spraying I've decided the answer is yes...and no.
The label on the pesticide says that it is most effective on annual weeds shorter than 6 inches. I wanted to see what difference weed whacking would make before spraying, so I used a string trimmer to cut most of the weeds to ground level. I also left one patch uncut.
I found that the herbicide appeared to affect the foliage of some weeds that were whacked that weren’t affected when left intact. However, weed whacking before spraying didn’t seem to make any difference for hardier weeds. Limonene appeared to cause foliage to burn on some weeds but it didn’t outright kill them; the affected weeds put out new growth within 2 weeks.
For example, garden spurge greater than 6 inches that was sprayed looked healthy and unaffected, however buttonweed foliage burned though the stems seemed unharmed. Nutgrass sedge seemed to be unaffected by any of the treatments.
Limonene seemed to affect foliage of some weeds quickly. Oxalis turned yellow immediately upon spraying, as did Drymaria. I mowed sleeping grass (Mimosa pudica) in the lawn, then sprayed it; the weed and the grass surrounding it turned brown, but new growth appeared within 2 weeks.
Okay, so all of this is anecdotal and not so scientific, but you’ve read this far so you might as well read the rest of this half-baked report.
Unless someone convinces me otherwise, I’m not using limonene as a weed killer again. I’m sticking to my electric string trimmer and propane flamer for weeds that aren't kept in check with mulching and hand-pulling. Spraying limonene was a pleasant, fragrant exercise in weed abuse, but certainly not worth $21.99 as a weed control strategy. My conclusion: When life gives you lemons, don’t make weed killer.