A really big herb farm

The below is not thoroughly tested, nor do I have a saved game in a good place for testing it. It might be impractical because it requires a lot of pruning. But if it does work, it could make a lot of self-sustaining herb patches in the big room.
Begin with a line of five herb bushes (for example, by adding two
bushes to the end of the common line of three), in the next generation,
they will grow to be a 3x3 block:


 .."..     .....
 .."..     .""".
 .."..  -> .""".
 .."..     .""".
 .."..     .....

Now prune two bushes to make a "pi" shape, and let it evolve for
five generations.

 .....                ..""...
 .""".                .."."".
 ...". -> -> -> -> -> ...."".
 .""".                .."."".
 .....                ..""...

Now prune two more bushes, and let it evolve.

 ..""...    ..""....
 .."..".    ..".."..
 ...."". -> ..."""".
 .."..".    ..".."..
 ..""...    ..""....

And two more, then another evolution.

 ..""....    ..""....
 ..".."..    ..""""".
 ....."". -> ....""".
 ..".."..    ..""""".
 ..""....    ..""....

And here is where it gets tricky. Four bushes must be pruned before
the next generation. If there's not enough time for hand-picking, fire
magic might be able to help.

 ..""....    ..""""...
 ..".""".    .."."""..
 ......". -> .......".
 ..".""".    .."."""..
 ..""....    ..""""...

And now four more herbs must be pruned before the next generation,
since they're all in a row, a fire bolt might be the best way here.
Then let it grow another generation.

..""."...    .."""""..
.."..""..    .."""""..
.......". -> .......".
.."..""..    .."""""..
..""."...    .."""""..

Another four herbs (but again, all in a row) must now be pruned.

.."".""..
.."".""..
.......".
.."".""..
.."".""..

The left half of the pattern is a pair of blocks that will stably
continue to blossom. The right half of the pattern is two generations
after the "pi" pattern, and in three generations will be ready for the
pruning sequence that led to its reproduction.
In a pacified Big Room, this could be used to make a double row of blocks all the way across the room. Something will have to be done to kill the pattern when it hits the far wall. If the Big Room is mined out, and if sufficient holy water is available to start the reproducing pattern described above three times in the right places, there is room for six rows of blocks, comprising at least one of each kind of herb and two of many.

A modification of the method by Darren Grey:

Also, it is possible to get the first five line-up from just a square of
four and one newly created bush. Goes as follows:

......    ......
......    ."....
.."".. -> ..""..
..""..    .."...
......    ......

Create one bush to the top left, and remove one from the bottom right. Then
leave for one generation and do the following:

......    ......
.."...    .."...
.""".. -> .""...
..""..    ..""..
......    ......

Now leave this alone for 8 generations and you'll get this:

........
...."...
..."""..
..".""..
."".."".
..""""..
..."""..
........

Some swift cutting can reduce this down to a line of 5 on the right. This
can be made a little easier by cutting down the step before this (the 7th
generation) like such:

.......    .......
.."""..    ...""..
."...".    .....".
."..... -> .......
.""."".    ...."".
..."...    ..."...
.......    .......

On the 8th generation this will then become:

.......
...."..
...."..
...."".
...."..
...."..
.......

So, overall, with a four-square, 4 holy waters (on average), and a bit of
patience, you can fill up the Big Room with herbs.
Yet another modification by Przemyslaw Brojewski:
Five generations is a long time gamewise, about 18 hours, or so.
A T-shape of herbs is a better point to start, because it is fairly common
to come by in the big room.

......    ......                                  ......    ......
.."...    ."""..                                  .."...    ."""..
.""".. -> .""".. prune two herbs to form a cross: .""".. -> ."."..
......    .."...                                  .."...    ."""..

There. Two generations, three bushes to prune and you got your Pi
shape.

[snip]
> And here is where it gets tricky. Four bushes must be pruned before
> the next generation. If there's not enough time for hand-picking, fire
> magic might be able to help.

>  ..""....    ..""""...
>  ..".""".    .."."""..
>  ......". -> .......".
>  ..".""".    .."."""..
>  ..""....    ..""""...

> And now four more herbs must be pruned before the next generation,
> since they're all in a row, a fire bolt might be the best way here.
> Then let it grow another generation.

> ..""."...    .."""""..
> .."..""..    .."""""..
> .......". -> .......".
> .."..""..    .."""""..
> ..""."...    .."""""..

This is indeed very tricky. Four herbs to prune between generations
is a lot. I wouldn't dare it without Burning Hands, especially if
my Herbalism skill was over 50. And the last pattern is very nasty,
if you don't manage to remove extra bushes, you may lose the whole farm.

I prefer this sequence:

........    ..."....    ..."....                     ..."....
.."""...    .."."...    .."""...                     .."""...
.."."... -> ."...".. -> .""."".. remove two bushes:  ."...".. ->
.."""...    .."."...    .."""...                     .."""...
........    ..."....    ..."....                     ..."....

.."""...                           .."."...    .."."...
.."""...                           .."""...    ."".""..
."...".. remove two bushes again:  ."...".. -> ."...".. remove one
.."""...                           .."""...    .""."".. herb bush:
.."""...                           .."."...    .."."...

.."."...
."".""..
....."..
."".""..
.."."...

And it evolves into this:

> .."".""..
> .."".""..
> .......".
> .."".""..
> .."".""..

From this pattern I remove four herb bushes, and it is the only
point at which I have to remove more than three[1] between generations:

.."".."...    ..""......
.."".."...    ..""..""..
.......".. -> ......""..
.."".."...    ..""..""..
.."".."...    ..""......

And after one generation I get a block of six herb bushes from which
I remove two to obtain (you guessed it) a T-shape.

[1] A T-shape may be positioned on the desired row in the big room just
    by removing extra herbs. It requires removal of three herb bushes
    between generations. I's been described in the guidebook.