Constructing a simple tree with ASCII in F#
There's a couple of simple things you can do to reduce the clutter in your code. For example, this:

let isEven = fun i -&gt; if i % 2 = 0 then true else false


Firstly,

if someCondition then true else false


can always be written simply as

someCondition


You're also missing a trick with your declaration:

let someFunction = fun i -&gt; ...


Can simply be written as

let someFunction i = ...


So your isEven function is simply:

let isEven i = i % 2 = 0


I'm coming back to F# after a long break (2+ years) so I'm not going to suggest anything else as I'm still relearning!

Edit:

Actually... 

let emptyRow = String.concat "" (Seq.map (fun x -&gt; "_") (seq{ 1..100}))


Is simply:

 let emptyRow = String.replicate 100 "_"