Tree farm inventory valuation

I can see two ways to value inventory for a tree farm: 1: What did it cost to get the tree to that size. To do this, I need to evaluate the price of the original pot, the seedling, the dirt, the
time to pot it, the watering and care, the transplant into the next size pot, the cost of that size pot, the average number of times I can use an intermediate pot, the cost of water, fertilizer, my time, the lost opportunity cost of money all for a multi year period. This is what I've been advised to do by a friend/bookkeeper. 2. The approximate retail value of the tree at it's present size. This I can evaluate fairly simply by going to Home Depot and seeing what trees of similar size/quality are selling for, and dividing by 2. (Usually my trees are better quality, and historically I've been able to get about 20% more than HD for this reason.)
Second question: The typical work flow to grow a tree goes like this:
Seedlings + styrofoam block pot + 1 year = super seedling. I sell these for $4 each. Any unsold by early August ...
Superseedling + 6 liter pot + dirt + 1-3 years = small tree. I sell these for $12 each. Any unsold by early August ...
Small tree + 20 liter pot + dirt + 2-3 years = medium tree. I sell these for $35 each.
Now when a seedling goes into a styropot it decreases in value: It's roots haven't filled up the pot, so it's difficult to transplant, A fraction die from transplant shock. Similarly a superseedling immediately after transplanting is undersized for it's pot, and so has a lower value. Normally I consider these 'not big enough for the pot' trees to be unsaleable.
So a given tree as a seedling is worth 50 cents. Intially in a styroblock it has value 0. A year later it has value $4. It's transplanted to a 6 liter pot. It again has either no value, or it disappears from my inventory for a couple years. Then for a while it's worth $12.
What is the best way to handle this in terms of the accounting?
I'm new to accounting. Assume lots of ignorance on my part when you answer.
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