Question
You suggested sugar to get rid of nutgrass.  Will the sugar harm my roses iris's?

Answer
I am sorry but you have got me confused with someone else.
Generally, with weed control, we are dealing with products which can kill plants. The danger lurks in the dosage. At a specified rate, the product will harm a particular weed but not a desirable shrub. Exceed that dose rate and all bets are off.
I have never seen sugar labeled for killing weeds and regretfully, I cannot offer any solution.