Making a desert into a green paradise

[lolcat]

Greetings playground!

As the title says, how to achieve that? The resources the party themselves have are an archivist/runecaster lvl 13, a Mage of the Arcane Order lvl 11, a healer lvl 13, a wilder/anarchic initiate lvl 12.
The size of the entire desert is about 800+400km (500*250 miles) or rocky/gravel desert and the area we care most about, about the center 2/3's of it, is pretty much devoid of all life.

So the approach i could think of at the moment is to create some huge runes floating high in the sky, activated by any creature reading them, having a rock to mud or create water be cast on the creature activating it. Then either bank on curious natives or have a permanent lesser planar binding active to get some water-elementals who should be happy to spread their element through the arid land without much or any compensation. Hopefully.
But even IF that works up to that point, it would only create a bog, without any vegetation. If i read it correctly, even "control plants" needs to have some initial vegetation to make that grow, so that's out in the majority of the desert which is pretty much all lifeless.

Any ideas on how to achieve this task? Bonus-points if it can be done in a short time-frame and is irreversible (the desert seems guarded / furthered by an avatar of destruction who seems rather fond of the place).

Thanks!

[Saint-Just]

Reminder that one of the weirdest parts of 3.5 is anhydrut - Inevitable specifically dedicated to saying "No" to attempts like yours. Single one is not a threat to your party but keep in mind that if cosmology is close to standard then that avatar of destruction may get significant support (and any plan that relies on non-combatants wandering the waste may be doomed).

[MaxiDuRaritry]

I could swear that there's a druid spell somewhere (possibly in Dark Sun?) that allows you to plant greenery in a wasteland desert that protects whatever you grow there, and it allows you to reclaim the blasted wastelands if nothing else manually destroys your efforts.

Can't find it anywhere, though. Anyone else know what it's called and where to find it?

[King of Nowhere]

how about some permanent form of control to keep it close to natural? they could trigger rain once per week or so

but hey, you can keep it even closer to natural: deserts are mostly formed by a rain shadow created by a mountain range. use magic to remove the mountain range, or at least open a large hole in it, and clouds will start moving over the desert on their own.

and once it starts raining, life will come. once wet, the land should be fertile, unless the desert also has problems akin to the great salt lakes (which are still teeming with life, but only on the microscopic level).
if that is the case, then it is caused by accumulated salts. it means there was once an ocean or large body of water that dried up, leaving salts behind. it happens because there is no water flowing out of your system and carrying away salts. water only flows in (bringing salts) and then evaporates, and salts get accumulated.
you can fix that by digging an outlet for your salt lake. now rain will bring water that will dissolve some salt and bring it to the ocean, and it's just a matter of waiting. although salt lakes are great places to mine lithium, you may regret washinbg all that salt in the ocean once your people discover modern batteries:smallcool: