How do I make old jeans stiff?

I bought a product called Stiffy but you will need much more than one of those bottles hold. Here is a home made recipe I found that you can increase:

1 Tbsp cornstarch, 1/4 C cold water, and 1/4 C boiling water.

Dissolve 1 Tbsp of cornstarch in 1/4 C cold water. Meanwhile, boil 1/4 C of water. Slowly, add the cornstarch solution to boiling water and whisk and boil until the solution bubbles. Take the solution off the heat and cool to room temperature before using.

The consistency is thick, like Tapioca pudding. It is translucent and dries clear.

Starch

Try Argo Starch. If it's anything like it was when I was a kid & my mother starched blouses, etc your jeans will stand up like if they were frozen on a cold winter day!

You can use a cement wash..mix up some cement.. very watery.. dip the jeans into it.. wring them out.. form them how you want them to shape up...and let them dry.

Make sure you get all the fabric covered...

I worked at a laundry and dry cleaners 10 years and the best thing you can do is get some old-fashioned liquid starch and soak them and then let them hang dry and iron them on a high heat

try using spray starch. I used it to stiffen my nursing caps. spray or soak in starch and dry them flat on top of a freezer.

while many suggested starch, or a similar product, I wonder how those would hold up long term with moisture for plants you intend to plant inside "still" jeans. Therefore, I'm thinking the cement dipping method may be a better choice!?