Our Plants Need How Much Water? Special Feature: Bad Math

August 18th, 2021

Dear Readers,

WARNING. This blog contains a lot of math in it. What’s worse is it is bad math. Unchecked math. Math that is most probably all wrong. Math done by me, Shelby. 

So, if you are math-savvy and have the potential to be utterly offended by the numbers I am just chucking out there…then I need your help. Well, more importantly, if you are a math master who can also speak to plants and convince them to drink as much greywater as possible…then I really need your help. 

I am also reaching out to our local 4H/Penn State Agricultural extension and plant-knowledgeable friends. Therefore the pressure is not all on you. But any extra help is always appreciated. Especially if you really can speak to plants… 

And! Without further ado…Welcome. To Bad Math.


Me being bad….with math.

We are estimating to produce about 30 gallons of kitchen and laundry effluent per day. 

30 gallons x 7 days = 210 gallons/ week

We need to efficiently plant our potential plants in the greenhouse/leach field in order to accommodate that number. 



For right now, let’s assume most of our plants need 1” of water per week. What is 1” of water translated into gallons? 

Apparently, if we had a one square foot (1’ x 1’) planter bed, that is 144 square inches (also, 144 cubic inches). There are 231 cubic inches in 1 gallon. Divide 231 by 144 = 0.62 gallons. So, a 1’ x1’ planter bed will need about 0.62 gallons per square foot per week.1

Let’s say we had a 3’ by 6’ planter bed. 3’ x 6’ = 18 square feet. 

With 0.62 gallons / square foot, we are looking at…18 x 0.62 = 11.16 gallons per week.

What if we have a plant that needs 2” of water per week? To begin, .62 gallons of water makes up 1 inch in a square foot.

Again, let’s say we have a 3’ x 6’ planter bed (18 square feet). 

First, multiply the .62 gallons of water for every inch needed (.62 gallons x 2 inches) = 1.24 gallons for 2” of water per square foot

Then, multiply the gallons for 2″ of water per square foot by the square feet of the planter bed: 18 square feet x 1.24 gallons for 2” per foot = 22.32 gallons of water per week. 




If .62 gallons of water make up 1 inch in a square foot, how many square feet of planter beds should we have? 

210 gallons of greywater produced per week / .62 gallons per square foot = 339 square feet of planter beds

That’s eighteen 18 square foot planter beds… 

We do NOT have room for that in our mudroom/greenhouse/leach field addition thing.

But, I thought…just maybe if we plant with space efficiency in mind, and we pick the most water-loving plants…we can make it work? The plants and how thirsty they are will make up for the lack of square footage we are willing to allocate toward this greenhouse?




And this is where things got messy….

Elderberries, for example, are recommended to have 1” of water per week.2 So I thought, “Great! If I plant two elderberries in a 2’ x 5’ planter bed, that’s 10 square feet.” 

10 square feet x 0.62 gallons for 1” of water per square foot = 6.2 gallons per week

Which sounds clear-cut and simple! But then I took a moment to [over] think…and I started wondering if it ACTUALLY was 1” of water per week per elderberry plant….huh. Ok. 


.62 gallons x 2” (because I have two plants) = 1.24 gallons for 2” of water per square foot. 

10 square feet x 1.24 gallons = 12.4 gallons per week

But that didn’t make much sense…that’s a lot of water…and also means each square foot is getting 2” of water…which is a lot…and not necessarily 1″ of water per plant?

So, I then found a resource that simplified the watering to giving each elderberry plant only one gallon of water per week!3 Phew. Well, that made the math simple….

2 elderberries x 1 gallon = 2 gallons per week




There of course was other research that went into the elderberry. Along with every…other…single…plant. Like, how far apart I have to space them and how deep and big do their roots/canopies grow and how tall do they get and are they self-pollinating and on and on and on and on…

In other words, as soon as William and I officially have our house designs roughly approved and we obtain funding, I will be hiring a professional to figure out a lot of this for me. Because, in truth, I have no idea how much water to give my elderberries per week. Or any of our other plants….Oh, I can do the good ol’ stick your finger in a bit and feel if it is dry trick…but that is not very official sounding when we bring our plans and calculations before code. 

I even tried to make an excel spreadsheet with actual calculations ‘coded’ into it…it didn’t work out too hot. And the designated gallons per week for every plant have changed much more than once…

Attempted excel spreadsheet. The plants in red are not included in the total amount of water per week going to the greenhouse. As of now, we do not have room for them. You’ll understand more when you see those floor plans in next week’s blog!

What I need from plant-speaking math masters is a way to reasonably estimate the gallons per week that would be going to this greenhouse. I know that number is dependent on a multitude of variables: the plant type, the health of the plant, the season of the year, what stage of growth the plant is in, how much sunlight does it receive…But I need a means to verifiably say “Yes, our greenhouse can act as a viable absorption field/ leach field for our kitchen and laundry greywater.” Without proof backed by numbers, I fear getting this experimental permit is going to be exceedingly more difficult.

Stinkin’ numbers. 

With that, I do believe this is where we shall end today’s Bad Math. 

Next week, you will get to see a floor plan or two of this mudroom/greenhouse/leach field thing!  

As always, thanks for readin’!


1. Drip Depot. “Inches to Gallons,” 25 Jan 2021. https://help.dripdepot.com/support/solutions/articles/11000100937-inches-to-gallons. Accessed on 16 Aug. 2021.

2. StarkBros. “Watering Elderberry Plants,” StarkBros Nursery & Orchards Co. https://www.starkbros.com/growing-guide/how-to-grow/berry-plants/elderberry-plants/watering. Accessed on 16 Aug. 2021.

3. Elderberry Edge Farm. “Growing the Elderberry,” 2021. https://www.elderberryedgefarm.com/pages/growing-the-elderberry. Accessed on 16 Aug. 2021.


  1. Shelby, you crack me up! No wonder William loves you, and no wonder we fell in love with you right from the start.
    But, I must say, the poor berries are wondering if they are going to drown or breath freely!! lol

    • Hey, Aunt Rose! 🙂 <3 hahaha William is certainly entertained by me to say the least! Lol , and yes, the berries are definitely going to be a subject of experimentation…I will not be able to say "no fruits were harmed in the making of this LBC home!" Because some will most likely endure some drowning…


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© 2020 Sustaining Tree