Halo Wars: How do I get the 9th Skull!?

[SkerlockHolmes]

I know that you kill 20 of the Flood Stalks... But I went through the whole mission looking for them and I never found them. Ive looked all over the internet for guides but there are none... I swear there is a guide for every other skull except 9...

if you know please help. Thank You

 

[Freak Me Apples]

You're Sherlock, you can figure it out yourself...