Math help region3

So basically I wanna spawn something randomly inside of a region3 but only on the Y axis to the floor; so how would I do that from where i am on this code

local plantsFolder = game.ServerStorage.plants:GetChildren()
	local regionPart = game.Workspace:FindFirstChild("defaultMap").plantSpawns.plantSpawn1
	local bottomLeft = regionPart.Position - (regionPart.Size / 2)
	local topRight = regionPart.Position + (regionPart.Size / 2)
	local region = Region3.new(bottomLeft,topRight)
	local function spawnPlants(obj,pos)
		local newPlant = obj:Clone()
		newPlant.Parent = workspace
		newPlant.PrimaryPart:SetNetworkOwner(nil)
		newPlant.PrimaryPart.Position = -- not sure
		print("Spawned "..newPlant.Name)
	end
	spawnPlants(plantsFolder[math.random(1,#plantsFolder)],--not sure)

I just had to do some cframe / vector math