By our calculations, you should look at sowing Sweet Wonder about 35 days before your last frost date.
Info from http://www.shootgardening.co.uk/plant/rosa-sweet-wonder?referrer=%2Fplant%2Fsearch%3Fq%3Dsweet+wonder%26amp%3Bsearch_submit_x%3D0%26amp%3Bsearch_submit_y%3D0