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[LMeire]

So I heard that cast obsidian yields sand floors now when mined-out, and it got me thinking: "Does this mean I can make farmscrapers now?"

But before I actually start up a world for the sole purpose of improbable agriculture projects, does anyone know if the cast obsidian still counts as above-ground after being "buried" by cooled magma? I'd like to know what kinds of seeds to bring for the endeavor. I'm looking forward to the possibility of a Dwarven Hanging Gardens, but I don't want to get my hopes up only to end up with a stockpile full of sunberry seed roasts.

[Alfrodo]

So I heard that cast obsidian yields sand floors now when mined-out, and it got me thinking: "Does this mean I can make farmscrapers now?"

But before I actually start up a world for the sole purpose of improbable agriculture projects, does anyone know if the cast obsidian still counts as above-ground after being "buried" by cooled magma? I'd like to know what kinds of seeds to bring for the endeavor. I'm looking forward to the possibility of a Dwarven Hanging Gardens, but I don't want to get my hopes up only to end up with a stockpile full of sunberry seed roasts.

1) yes, when a tile is exposed to above ground it stays that way.

2) Could you not make arable mud floors on built structures with buckets and clever pump stacks before?

3) If you want to go for a dwarven hanging gardens, you could find a way to get grass up there, and have persimmon trees grow. Otherwise I'd go with potato for all purpose crop, radish or turnip for masochist dwarves or sadistic overseers, and hemp or jute if you're up for a clothing industry.

[PatrikLundell]

I don't think mining obsidian yields sand floor, at least not consistently (there are some oddities with muddied floor that sometimes yield sand or soil when the tile is floor/walled over and the wall is removed).
However, as Alfrodo said, you can muddy ordinary floor tiles and make farm plots on top of them, and if done above ground (i.e. tile exposed to sunlight at some time) you'd get above ground crops as plantable choices.
I don't know if "wild" plants grow on muddied floor, and, if they do, trees would grow there. Saplings can mature into trees even if the soil beneath has been mined out, but I'm not sure if saplings will start to grow without any soil beneath.

If you do some !science!, let us know.

[Bumber]

I don't think mining obsidian yields sand floor, at least not consistently...

Above ground?

[§k]

Tiles over ground probably don't support plant. I've casted and muddied obsidian pillar in .34, but no plant appeared after years.

[PatrikLundell]

Muddied above ground floor supports farming, but probably not "natural" growth.

[LMeire]

I was under the impression that indoor mud would be cleaned up if it was aboveground like any other contaminant, at least that's what the wiki seems to imply. I'm still gonna try it with magma though, for !!SCIENCE!! if nothing else.

[PatrikLundell]

I've done some experimenting with obsidianization and then channeling above ground.
The first one was a two tile high obsidian pillar where I channeled away the top layer. The floor remaining was reported as being conglomerate, which matches what was below the bottom obsidian tile (in an ocean biome).
The second one was on a constructed floor on top of my recessed courtyard, and just a single obsidian block. Channeling away the block yielded an unusable slope (due to how I channeled, nothing strange), and when the ramp was removed I ended up with black sand, which matches the material of the ground in the area, as well as the ground in my courtyard.
My current hypothesis is thus that obsidian casted and channeled away above ground yields the material of the ground below, but the sample size is a bit small. Whether anything will grow there naturally remains to be seen (nothing is growing on the muddied floor in the tile beside it, as expected). I'll have to wait a few years...

Edit: That was quick: Spare sawgrass. We'll see if herbs or saplings grow as well. At least you can create artificial grazing grounds...

[Alfrodo]

Welp.

I guess next time I play DF I'm going to make the hanging gardens of Babylon.

Also, is the sawgrass growing on just the sand or the mud as well?

[PatrikLundell]

The sawgrass is NOT growing on the mud (and it's been muddy for a number of years), but (now) both on my floor (where I channeled away an obsidian block) and on top of another obsidian block where i channeled away a top one. The second patch had silty clay loam on the surface layer with black sand underneath, and was displayed as black sand when the obsidian was channeled.
I'm slowly working on increasing the sandy floor area to see if herbs/saplings/trees will grow there. I might increase the size of the obsidian pillar if I get herbs on the floor (to see if rock underneath sand works better for saplings/trees than floor underneath the sand).

[Alfrodo]

huh.

So the only way to make a PROPER hanging garden is with an obsidian glitch.

Mud can still be used for farming in high elevations (what OP wanted) but if you want grass and persimmon trees, you'll need lava, lots of scaffolding / casing, and a pump stack.

[PatrikLundell]

Well, I still haven't seen anything but grass. I'm slowly obsidianizing an area using mine cart hauling+bucket brigade. When I'm satisfied with the amount of obsidian I'll channel it down and see what happens (I AM building a magma pump stack, but I've still got about 110 levels to go, and I'm in the process of making a pump stack worthy puncture in the aquifer, so the gardening experiment should be done well before the pump stack is).

[Loci]

So I heard that cast obsidian yields sand floors now when mined-out

Not quite. When Toady nerfed golden floors he made all mined-out tiles revert to the layer's base material. If you cast obsidian on a gabbro layer, then mine it out, you'll get a gabbro floor. If you cast obsidian on a sand layer, then mine it out, you'll get sand. Even "sky" levels have a base material, usually some type of soil, so mining it out will often yield a soil floor capable of growing crops, though not necessarily sand.

I'd like to know what kinds of seeds to bring for the endeavor.

Since the above-ground tiles will remain sunlit and dwarves cannot embark with above-ground seeds, you'll need to gather plants or trade for seeds after embark.

[PatrikLundell]

Actually, I toss out the plump helments and bring garden vegetables on embark instead, a few of each brewable version available. Those will then yield seeds when brewed/eaten raw. Whether those seeds are usable on the embark depends on biome compatibility. I then complement this by gathering all the herbs I can in the embark each season for the first year to try to get all kinds (but can still miss some).

While there is nothing to gather in deserts without any ground cover, you can still farm in them (and if you channel out the top level of a multi level soil embark grass will grow to support grazers, but no herbs or trees). Mountain biomes do no support above ground farming, and glaciers don't either (of course).

Update: I just found Whip Vine on my obsidian->sand floor, so you can apparently make a hanging garden (or camouflage your fortress). I'll continue to wait to see if you can get saplings as well. The finding is encouraging enough to expand the area, and to try  a separate sand on obsidian area (currenty that one is a single tile only).

[PatrikLundell]

I have to revise my results: Obsidian channeled away does not seem to allow herb growth in the sky.

I've made a few experiments:
- Channeling away the ground for a recessed courtyard, building a floor on top of that, covering a 5*11 area with obsidian that's then channeled away.
This setup resulted in a ground of mostly black sand, with 5 tiles of obsidian floor in it. It can be noted that obsidian pebbles are "native" to the area, with black sand dominating. This setup has produced a total of 3 herbs over a 3 year period while the area grew from a 3*3 size. No saplings have been seen. This seems similar to the anemic vegetation found on ground where the layer below has been channeled away, so only the soil "floor" remains. I cannot rule out that this environment might support saplings, since non grass vegetation is so rare. I've seen saplings mature in a "floor" environment, but cannot say if new saplings can spawn there.
- A two tile high obsidian pillar shaved down to 1 z level in the ocean biome resulted in a conglomerate floor, matching the ocean shore material underneath.
- A two tile high obsidian pillar shaved down to 1 z level in my land biome resulted in black sand that's been covered by grass for 3 or so years.
- A two tile high block sized 5*11 shaved down to 1 z level in my land biome resulted in a uniform layer of black sand (the ground beneath had 2 obsidian pebble tiles). This area has now been covered by grass after one year, but neither herbs nor saplings have been seen. I cannot rule out that this is an anemic "floor" environment that might eventually produce a herb or two, but I certainly wouldn't bet on it.
- As a control, I created a dirt road over a 5*11 area. This area is covered by grass with 4 saplings a a number of herbs after 1½ years.

And finally, a question of my own: My embark does not have any native pools. In an attempt to acquire water loving herbs (sun berries, rope reed, fisher berries, etc) that exist in the region but not on my embark I created a 3*3 pool, and I've destroyed all "normal" herbs and saplings appearing around it (to free up the room) for 3 or so years. I've lured a pond turtle there, but that's obviously not a herb. Does this strategy have any chance of succeeding?