A lack of patience.
Seriously, they breed like weeds for me, and it's almost to the point where I'm getting a surplus of them.
Besides using the golden watering can and fertilizer, as Skweekerz does, try TT-ing. Water them and plant the fertilizer, then TT forward one day. Check to see if one spawns. While you're a day ahead, buy more fertilizer, and TT back to the day before. Rinse and repeat; usually if none spawn, keep trying. They will eventually. Once you have more though, plant them in a checkerboard pattern:
x o x o x
o x o x o
x o x o x
o x o x o
x o x o x
(x = blue rose; o = empty spot/buried fertilizer)
Making sure to water them all and plant some fertilizer, too. Usually, for me, I have 20 to 30 blue roses planted in a checkerboard pattern in an area, and if I water them everyday, I usually get 3 to 5 a day without using any fertilizer. It took like a few weeks though to get enough to have a large enough area, but once they're established and you breed a ton, they'll pop up like weeds.
