You'll need to earn stars to open up new worlds. In this particular star challenge, you'll need to keep the zombies from crossing the line of flowers in order to get the star.
Caption byJason Parker
/ Photo by Screenshot by Jason Parker/CNET
You can also look at the almanac to see the possible upgrades. Notice that the Sun Bonus (for purchase) does the same thing as the Sun Boost (unlockable), but you'll have to pay for the Sun Bonus to get 50 sun bonus at the start of a level instead of just 25.
Caption byJason Parker
/ Photo by Screenshot by Jason Parker/CNET
The Snow Pea was a plant you unlocked early in the original game, but in Plants vs. Zombies 2 you'll need to buy it. Fortunately there are not that many plants that are for purchase only.
Caption byJason Parker
/ Photo by Screenshot by Jason Parker/CNET
Some plants for purchase are merely enhancements (like the Imitater), but old staples from the original game such as the Squash, Snow Pea, and Jalapeno are behind the pay wall.
Caption byJason Parker
/ Photo by Screenshot by Jason Parker/CNET