On Master Level ?

@Robinson270 Here a list of the max level for each world plant:

20 Level World Plants:

Peashooter, Wall-nut, Potato Mine, Cabbage-pult, Bloomerang, Iceberg Lettuce, Bonk Choy, Repeater, Threepeater, Split Pea, Puff-shroom

15 Level World Plants:

Kernel-pult, Spring Bean, Pea Pod, Lightning Reed, Melon-pult, Pepper-pult, Laser Bean, Fume-shroom, Phat Beet, Spore-shroom, Primal Peashooter, Primal Wall-nut, Guacodile

10 Level World Plants:

Sunflower, Grave Buster, Twin Sunflower, Snapdragon, Spikeweed, Coconut Cannon, Spikerock, Cherry Bomb, Chili Bean, Tall-nut, Winter Melon, Hot Potato, Chard Guard, Stunion, Rotobaga, Red Stinger, A.K.E.E., Endurain, Stallia, Blover, Citron, E.M. Peach, Infi-nut, Magnifying Grass, Tile Turnip, Sun-shroom, Sun Bean, Magnet-shroom, Celery Stalker, Garlic, Intensive Carrot, Perfume-shroom, Primal Sunflower, Primal Potato Mine, Lily Pad, Tangle Kelp, Bowling Bulb, Banana Launcher, Moonflower, Nightshade, Shadow-shroom, Dusk Lobber, Grimrose

Once the plant reached max level you can upgrade it to Mastery.

@Samsung_Tab_III 

Some seeds are harder to come by than others.

@Perfume-shroom92 

I don't know, i want to max out bowling bulb and lightning reed.

from my observance,primal peashooter is the hardest lv15 plant to max out. 

@Perfume-shroom92 

I have always liked lightning reed but he's not powerful enough for Arena.

His seeds are still hard to come by though.

There are still several other plants that seeds are hard to come by, and some plants (especially premiums) seeds are plentiful. 

@Perfume-shroom92 

I'm probably going to buy wasabi whip and cactus and i expect them both to go into mastery when i get them.

I'm going off now to make some cheese scones or something now.