Urea can be used, but you want to water WELL, first. Nitrogen burns. It's ammonia. Before it can be safely used on roses, they must be full of water first, so when the "saltier" water from the dissolved urea sucks out the distilled water from the roots and replaces it with the nitrogen salts rich water, it doesn't burn the plant. If you need that extra nitrogen, then wait until after a good rain (I know you don't have irrigation and must haul water) then lightly spread it around away from the trunks of the plants so the next rain can rinse it through the feeder roots. Hopefully you will have two rains you can use to apply it between so there is enough water to prevent the salt burn from the urea. And, as gardengal recommended, use it VERY sparingly. It's potent, "salty" stuff! Fingers crossed for you!