Example

// Your plant code should look something like this

public static class BlockCustomFlower extends BlockFlower {
        public BlockCustomFlower() {
            setSoundType(SoundType.PLANT);
            setCreativeTab(CreativeTabs.DECORATIONS);
            setHardness(0F);
            setResistance(0F);
            setLightLevel(0F);
            setUnlocalizedName("ocotillo");
            setRegistryName("ocotillo");
        }

        @Override
        public BlockFlower.EnumFlowerColor getBlockType() {
            return BlockFlower.EnumFlowerColor.YELLOW;
        }

        @SideOnly(Side.CLIENT)
        @Override
        public void getSubBlocks(CreativeTabs tab, net.minecraft.util.NonNullList<ItemStack> list) {
            for (BlockFlower.EnumFlowerType blockflower$enumflowertype : BlockFlower.EnumFlowerType.getTypes(this.getBlockType())) {
                list.add(new ItemStack(this, 1, blockflower$enumflowertype.getMeta()));
            }
        }
        // Up to here it's just the default MCreator code

        // This plant grows on sand, (stained) terracotta and dirt
        // You need canSustainBush, canPlaceBlockAt and canBlockStay to make it work properly
        // Don't worry about imports, just press ctrl+w after pasting the stuff from here...
        @Override
        protected boolean canSustainBush(IBlockState state) {
            return state.getBlock() == Blocks.SAND || state.getBlock() == Blocks.HARDENED_CLAY || state.getBlock() == Blocks.DIRT
                    || state.getBlock() == Blocks.STAINED_HARDENED_CLAY;
        }

        @Override
        public boolean canPlaceBlockAt(World worldIn, BlockPos pos) {
            IBlockState soil = worldIn.getBlockState(pos.down());
            Block block = soil.getBlock();
            return block == Blocks.SAND || block == Blocks.HARDENED_CLAY || block == Blocks.DIRT || block == Blocks.STAINED_HARDENED_CLAY;
        }

        @Override
        public boolean canBlockStay(World worldIn, BlockPos pos, IBlockState state) {
            if (pos.getY() >= 0 && pos.getY() < 256) {
                IBlockState iblockstate = worldIn.getBlockState(pos.down());
                Block block = iblockstate.getBlock();
                return block == Blocks.SAND || block == Blocks.HARDENED_CLAY || block == Blocks.DIRT || block == Blocks.STAINED_HARDENED_CLAY;
            } else
                return false;
        }
        // ...to here
    }
}

// Say you want to make the plant grow on a block you created (for example a ruby block)
// You need to know the name of your block element (the one you pick when you created the block)
// Let's say the name of the ruby block is RubyBlock
// In this example, the plant grows on the custom ruby block instead
public static class BlockCustomFlower extends BlockFlower {
        public BlockCustomFlower() {
            setSoundType(SoundType.PLANT);
            setCreativeTab(CreativeTabs.DECORATIONS);
            setHardness(0F);
            setResistance(0F);
            setLightLevel(0F);
            setUnlocalizedName("ocotillo");
            setRegistryName("ocotillo");
        }

        @Override
        public BlockFlower.EnumFlowerColor getBlockType() {
            return BlockFlower.EnumFlowerColor.YELLOW;
        }

        @SideOnly(Side.CLIENT)
        @Override
        public void getSubBlocks(CreativeTabs tab, net.minecraft.util.NonNullList<ItemStack> list) {
            for (BlockFlower.EnumFlowerType blockflower$enumflowertype : BlockFlower.EnumFlowerType.getTypes(this.getBlockType())) {
                list.add(new ItemStack(this, 1, blockflower$enumflowertype.getMeta()));
            }
        }

        // This plant grows on ruby blocks
        // I replaced the Blocks.SAND, Blocks.DIRT etc. with MCreatorRubyBlock.block
        // In general, for any block element called "BlockElement", you can get the block with "MCreatorBlockElement.block"
        // Again, don't forget to press ctrl+w after copying this stuff!
        @Override
        protected boolean canSustainBush(IBlockState state) {
            return state.getBlock() == MCreatorRubyBlock.block;
        }

        @Override
        public boolean canPlaceBlockAt(World worldIn, BlockPos pos) {
            IBlockState soil = worldIn.getBlockState(pos.down());
            Block block = soil.getBlock();
            return block == MCreatorRubyBlock.block;
        }

        @Override
        public boolean canBlockStay(World worldIn, BlockPos pos, IBlockState state) {
            if (pos.getY() >= 0 && pos.getY() < 256) {
                IBlockState iblockstate = worldIn.getBlockState(pos.down());
                Block block = iblockstate.getBlock();
                return block == MCreatorRubyBlock.block;
            } else
                return false;
        }
    }
}