Need help building wooden window box planters

I have 3 large window boxes (permanent, bolted to the outside of the house) and need help to build customized planters to go in them. Approximate size of the window box is 10" D by 80 inches wide and I am looking to make 8H x 8D x 75W" planters (3 planters total). The idea is the longer the planter the more room there is for roots to grow for Annuals. I am unable to decide on the best kind of wood for this project. Appearance is not important as the planter is going to sit inside the window box. I checked HD and they got Doug Fir hardwood with 1" thickness. While the wood is very strong and looks beautiful but Its very heavy especially for a 6 feet long planter. Is there any other wood i can get at HD that would be good for outdoor use yet lighter?

One alternative is to use pressure treated 1/2" but i am getting conflicting opinions because of damaging effect of toxic chemicals on plants. Can i use a wood sealer on the inside of pressure treated wood? I have included a picture of what i am trying to build.

Any assistance would be appreciated.

 

No matter what type of wood you use it would be best to install a liner to protect the wood from moisture.

 

If the window boxes are already there I would make galvanized steel liners. Making wood boxes to go inside the existing wood boxes would further reduce the space available for soil and you would still need to line your liner box to protect it from the moist soil.

 

Pressure treated chemicals are not a big worry these days. Some are inert. Is there a roof overhang to keep rain off the boxes ? There are 8 foot x 1/2 inch x 5 inch wide, cedar fence slats that you could cobble together to line the inside of your existing planters. Liners are a good idea, but you can also drill holes for drainage.....What wood is the existing planter, how old is it ?

 

Yes the window box is already there. I am not sure what wood is existing planter. The planters are very old but still in good shape (house is about 100 yr old). I have attached a picture that i had from last year for reference (note the window shown in the pic is smaller but the side windows are larger) There is not much of an overhang, especially on the side windows and rain water can easily get in.

Looks like the consensus is to use existing planters with liners. The reason i didnt want to go that route was I am in NY and in winter i can easily take the planters (that i was going to build) down for storage and the next spring i can start fresh with new potting soil.

Now if i were to line the existing planters and put the soil in, can i reuse the soil next season? Would the weight and moisture from the soil year-round damage the planter? What care should i take in winter if i decide to keep the soil in there all year?