Yellow mums are a growable flower in Animal Crossing: New Horizons.
This flower can be grown from seeds, which are sold at Nook's Cranny and by Leif. Nook's Cranny on each island will only stock three of the nine varieties of seeds (each in three colors, but only 2 colors each day), while Leif will sell all kinds of seeds (3 varieties per visit). To obtain seeds for your non-native flowers, wait for Leif or visit friends' islands.
There is also a special, hybrid version of this flower which can only be obtained via breeding. In the game, it looks exactly like the non-hybrid version and does not contain 'hybrid' in the name. However, it will breed differently than the regular version.
This item is used as a crafting ingredient in 4 DIY recipes:
You can see our full flower breeding guide here.