Like a hearty tree slowly but steadily growing with each passing year, Plants vs. Zombies HD adds another ring to the series. This new instalment expands the inventive tower defence gameplay to a more spacious screen ideal for its flora-on-undead-fauna action.

Yet along with growth rings come knots, too. While it remains just as playable on iPad, the addition of new content raising the price significantly higher than the iPhone and iPod touch version cuts into your wallet a little more.

Botany meets lobotomy on the home front - an infestation of zombies forces you to take up botanical arms in your yard and roof. Planting dangerous varietals like Pea Shooters, Cherry Bombs, Fume-Shrooms, and Squash enables you to defend against the advances of comic zombie foes. Moon-walking undead, Zamboni-driving zombies, and even sneaky snorkelers have to be outsmarted.

High-definition horticulture

You do this by plopping down plants on a grid, the right side of which enter the zombies. Preventing them from reaching your house on the left side is your paramount goal. A column of lawnmowers serve as your last line of defence should any zombies chew through your landscaping, but if those are gone when an enemy reaches your steps it's Game Over.

Adventure mode chronicles your effort to put the undead to rest through a lengthy 50 stages, all of which are unlocked for one-off Quick Play upon completion. Mini-games are sprinkled throughout to keep things fresh, such as Wall-Nut Bowling in which Wall-Nuts come in via conveyor belt at the top of the screen and you flick them onto the yard to knock out any marching zombies.

iPad even hosts a new mini-game, Buttered Popcorn, where a generous slathering of butter is applied to undead combatants before they get blasted by Corn Cob Cannons.

Money doesn't grow on trees

It's all such great fun that the addition of Survival mode and the Buttered Popcorn mini-game only enable what is sure to become an unshakable gameplay habit. Plants vs. Zombies HD possesses that rare combination of accessible and challenging gameplay that grows on you from the moment you plant your first defence. That iPad gets a little more content makes it all the more appealing.

Of course, these additions raise the price significantly - by a factor of three. The decision whether to buy this high definition port or the iPhone version comes down to a matter of control and content. High resolution graphics are nice, but not the best reason for spending a few extra bucks.

On regular stages the iPad version feels better because of the larger screen, which naturally grants more space for arranging your plants and scouting incoming zombies. Mind you, the number of rows and columns in each stage remains the same - they're just larger.

Best of all possible worlds

When it comes to the mini-games, however, I prefer the iPhone version. Wall-Nut Bowling, for example, has the conveyor belt situated horizontally across the top of the screen: I like it vertical to the left, as on iPhone and iPod touch, so I can slide a finger from the menu to the spot I want the defence planted without covering up the so much of the screen. An option to reconfigure the interface would be ideal.

Lastly, the re-introduction of Survival mode deserves an increase in price, though the continued omission of Zen Garden for cultivating extra money needed for unlocking the most expensive plants is disappointing.

To be fair, Plants vs. Zombies HD cultivates the same quality gameplay on iPad as it did on iPhone and iPod touch. More content on iPad does justify the price, though if you've already experienced the wrath of the Yeti Zombie then think twice before digging into this high definition release.