Please see the attached link. It recommends applying before having 2-70 degree days.

This is my second year of applying. I did it last year and didn't lose any to borers.

Here is a link that might be useful: MERIT to prevent borers

Thank you so much!!! I don't know where you are, but we in Chicagoland have had an historically warm April and now in May it has been cool - even nearly freezing two nights ago. I am going to apply it anyway - can't hurt. And of course as this great info advises, check the leaves and crush them before they get to the rhizome - if it's not too late. Thanks again - appreciate all that information you attached.

Westy

I'm in RI. We've had pretty much the same weather as you. Freeze and frost warnings the previous 2 nights! ARGH!!!

Thankfully I'm within the city limits so we only got into the low 40's both nights but it sure has kept the iris buds from opening.

Just out of curiosity, where can you buy Merit at in a small quantity? Thanks