Never quite sure whether it was a day to celebrate your own mum, or a day for all mums. We've added it without the apostrophe and I am quite sure HelpMeFind's search facility can cope with that. Yep. checked. It can.

Are you sure the plant is receiving sufficient water? I deal with two Margo Kosters, which are parallel sports of the same rose your Mothersday is. Neither does well here north of Los Angeles (also a higher heat area, like yours) unless I blow them out with a strong stream of water from the hose periodically. One is a bush, the other a standard. Both have greatly improved foliage of more lush appearance, better color with significantly more bloom of more appropriate color with the baths and increased water. If your plant is a more recent transplant, that extra hydration will also help it establish itself faster. These roses originated in much colder, wetter climates than either of ours. They really seem to appreciate a steady supply of water with regular 'baths' they would normally receive from rains. They seldom get rains here.

Also, how close is your plant to any hardscape? Walls, side walks, drive way, landscape boulders, etc.? It's very easy to bake, literally cook plants with reflected, radiated heat from these solar collecting surfaces. Those effects can be significantly greater for shorter plants as well as plants which aren't as well established in their positions. I'd check out the potential water and reflected, radiated heat issues before applying anything other than water. I would also try giving it a good shower every week during the higher heat and see what effect it has. The extra water suggestion is given based upon the presumption the plant grows with sufficient drainage.