Well, you need a lot more to make stuff grow than just seeds and a bit of dirt. Every farmer will tell you that. You need an ecosystem and a climate that can support the plants. If they don't exist, you need to make that first, and that is a process that would take a lot of very complex planning and environmental engineering, not just dropping seeds on the ground...
What you could do instead (well, not on all planets, but it could work for Mars) would be to use real bombs. Enough of them would heat up the atmosphere, releasing CO2 trapped in polar ice caps (pretty specific to Mars, I guess) which would heat things up even more.
Not a very effective strategy, but I guess it could work...
If bombing places with seeds was a highly effective strategy for farming, reforestation and creating/recreating/restoring ecosystems, it would have been adopted as a practice decades ago.
Unfortunately, even on Earth, in environments conducive to the growth of forests, things are a whole lot more complex than this.
And another problem is the environment of the planet in question- there are different requirements for terraforming Mars than there would be for, say, Venus. You might be able to make a planet habitable by dropping things on it, but the question of how you're dropping those things, where you're dropping them, what exactly it is that you're dropping, what you expect it to do, and how you expect it to do it, are most important. And they are very big questions to ask.
On Mars, it might be possible to get CO2 ice on the polar caps to sublime, increasing atmospheric pressure and temperature on the surface, by dusting the caps with dark Martian regolith to reduce their albedo. This might be the first step toward a partially terraformed Mars- one with more hospitable temperatures and more survivable pressures- but a 'fully terraformed' Mars on which a functioning ecosystem can be grown would still be a fairly far away goal.
I would not suggest trees, but rather simple [ame="http://en.wikipedia.org/wiki/Pioneer_organism"]pioneer organisms[/ame], hardy species that can colonise barren environments.
In the case of Mars, algae and lichens are often suggested, but organisms used for terraforming may be of other sorts- they may even be vascular plants, for example. But they would probably have to be engineered for the harsh environments that could be found (and also engineered to turn those environments into more livable ones for other species). The nature of the needed adaptions depends on the nature of the conditions on the planet in question. In the case of Martian terraforming these harsh conditions could include;
1. High levels of CO2.
2. Fairly extreme low temperatures.
3. Lack of water.
4. Extremely salty soils, or an abundance of metal ions.
Since a planet during terraforming has not been subject to the actions of a functioning hydrosphere or ecology in the recent past, many aspects of Earth's chemical environment that we take for granted would be absent. A terraforming pioneer organism would have to deal with this environment, and convert it into a more livable one.
It would probably be useful to have an organism that reproduces rapidly, and also probably useful to have some sort of kill-switch engineered into the organism (i.e. a weakness to a specific pathogen or chemical), if exterminating it en-masse becomes necessary.
Rather than spending effort 'bombing' the planet with pioneer organisms and fertiliser, it's probably a better bet to conspire so that organic materials spread across the planet 'automatically'.