The Queen of Thorns isn't terrible, not by any means, but she certainly feels underpowered when compared to the other Demigods.I feel that some adjustments are in order.

Most suggestions will be an either/or sort of suggestion. In other words, not all of the ideas need to be combined to fix the character as doing so would probably unbalance the character.

Shamblers:

• They seem to have AI issues.
• Their mana cost is far too high
• They are too weak for their mana cost

Solutions:

• Boost their damage output
• Reduce their mana cost
• Increase the amount of shamblers summoned and increase the max limit on shamblers by 2x (or maybe 1.5x)
• [EDIT] Boost their health! This is a must for Shamblers to be of any use whatsoever.

Bramble Shield:

• The ability has scaling issues.

Solution:

• Make the shield weaker early on and strong later. This change should be pretty light (approx. 50 to 100 points)

Spike Wave:

• Damage is too low for mana cost
• Speed reduction is too low for mana cost

Solutions:

• Increase the damage output
• Increase the speed debuff

*Note: Either option would be suitable, but both would be overpowered.

Compost:

• It's only worth taking if you take Uproot, which isn't very useful.

Solution:

• Change compost to so that it increases both health and mana regeneration rates for the Queen and her minions.

Uproot:

• It becomes less useful as the match progresses (when there are less/no structures)
• It has no impact on other Demigods.

Solution:

• Redesign the ability! I suggest calling it Strangling Vines; the damage should be lowered and the ability would target both Demigods and Structures. It should have a 1% chance to stun for 0.3 seconds every time it ticks. 

Morale:

• Does anyone actually use these?
• The Assassin version is far superior

Solution:

• Grant some sort of benefit to the Demigod themselves; not as much as the Assassin version though.
  

Ground Spikes and Entourage seem fine, although I am uncertain abou the usefulness of Tribute.