2 block Cactus (or higher) dont broke with water,but 1 block cactus can.
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In the next images we can see the diferences between 1 and 2 block cactus.
1) Cactus of 1 block breaks with water
2) Cactus of 2 blocks dont broke
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If this bug its fixed, the comunity can desing new and better cactus farms.