There are a few ways I could see you going about this. You could make your vegetation object and dynamically scale it up. You could animate the leaves and stuff to sort of "unfold", if your plants have enough polygons to be animated. If you are using UDK, you could very easily use the material editor to make a nice animation using the same principle as the dissolve effect.
If you wanted to make a blade of grass, for example, you'd start with something like the image below and use the UDK material nodes (scaler parameter, multiply, power, and clamp) and plug it into the opacity to animate it growing, sort of like I did in photoshop.
EDIT: As you can see, with a cleverly painted map, you can use this technique to produce a pretty convincing result.
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If you wanted to make a blade of grass, for example, you'd start with something like the image below and use the UDK material nodes (scaler parameter, multiply, power, and clamp) and plug it into the opacity to animate it growing, sort of like I did in photoshop.
EDIT: As you can see, with a cleverly painted map, you can use this technique to produce a pretty convincing result.