the three-peaters are more about space management. a column of three-peaters does 3 damage per row (2 at top and bottom) which doubles to 6 (4) with another column and 12 (8) with torchwoods vs. way less damage per column for every other plant type save gatling peas and watermelons (the latter of which i consider less reliable, they do 3 damage plus 1 splash to anything in a cherry bomb radius, but that splash is really hard to account for, as a mortar it takes a long time for the watermelons to travel, and torchwoods don't double their damage). on world 5 the only enemies that ever made it past column 9 were imps and bungies with even gargantuans dying to this plus a squash (and timed correctly thy shouldn't throw their imps). like i said i imagine this is not cost effective on one flag levels where you're not reaching the 16+ sunflowers which i agree seem to be the optimal amount for longer levels, but otherwise i find this the only way to deal with losing so much space to sun, especially on roof levels where you can't plant anything that isn't a mortar further back than column 5.
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