Making Planters

I am organizing a service project where I make these planters high for assisted living center so that residents do not need to stoop to their gardening.

What type of wood would be best for this? I do not really know what are the options for this type of project.

sequoia

The best wood is the wood of the bridge. It is made of recycled plastic and wood chip composite. It will last 100 years. without decay. Other wise, cedar or redwood is best. normal wood will rot in five years, pressure-treated wood is poisonous and carcinogenic chemicals used in eliminating mold and parasites.

Dido on the response of the other guys. I build a loft bed as we speak and I am using the terrace. You do not want to use anything pressure treated. They may try to tell you at the warehouse of pressure treated wood is more flexible pest; Do not believe them. Recycled plastic wood decking / survive almost anything else and it is easy to work with. In addition, it has the advantage of not being made from endangered species.
If you want a bed of slightly more than six stacked plates are sure to be close.

I am afraid of treated wood, because it could contain arsenic or other harmful chemicals. Cedar or redwood would be my choice, but it is expensive.

if you can get black locust, often used for fence posts, it will last longer than steel ...