First find the total number of petunias: 4 flats * 8 petunias/flat = <<4*8=32>>32 petunias.
Then find the total number of roses: 3 flats * 6 roses/flat = <<3*6=18>>18 roses.
Now find how much fertilizer Seymour needs for the petunias: 32 petunias * 8 oz/petunia = <<32*8=256>>256 oz.
Now find how much fertilizer Seymour needs for the roses: 18 roses * 3 oz/rose = <<18*3=54>>54 oz
Now find how much fertilizer Seymour needs for the Venus flytraps: 2 Venus flytraps * 2 oz/Venus flytrap = <<2*2=4>>4 oz.
Now add up the fertilizer for all three plants: 256 oz + 54 oz + 4 oz = <<256+54+4=314>>314 oz.
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