How to Make a Safe Herb Garden Within Creosote Planter Box

If it were me I use a belts-and-suspenders approach: I'd line the planter with the plastic sheet material used for backyard ponds (with some holes in the middle for drainage) then I'd place plastic or clay planter boxes inside the planting bed with mulch under and around them. Keep out the weeds, burrowing critters, insects and toxic chemicals.

Creosote can kill or harm plants exposed to it, if it is heavy as you say. Fresh creosote gives off some fumes as well especially in hot sun, so the plants may be affected if they are close to the ties. And, since some constituents of the creosote are water soluble, heavy rain could also splash toxic stuff onto your edible flowers, which is neither great for them nor for the eater.

So in addition to usb's suggestion I'd say, position those pots as far as possible from the ties, far enough to be certain they won't be splashed. Or, use the RR planter for something ornamental but not terribly expensive and rare (just in case!), and put your food somewhere else.

Re: Creosote can kill or harm plants exposed to it, if it is heavy as you say. Fresh creosote gives off some fumes as well especially in hot sun, so the plants may be affected if they are close to the ties. And, since some constituents of the creosote are water soluble, heavy rain could also splash toxic stuff onto your edible flowers, which is neither great for them nor for the eater.

If you use some of the pond liner type material as suggested by Usbport, try lapping it over the ties to prevent the potential for splashing and keep fumes to the underside and sort of directed away from the plants. Then I'd cover the material with a layer of soil, gravel, or mulch to minimize exposure to the sun--I'm sure minimizing solar exposure will prolong the life of the material.

BTW, I have used treated lumber (both the green stuff and the creosoted stuff) to make borders around things like flower beds--I haven't noticed a problem with the flowers growing.

Re: creosote can kill or harm plants: I am reporting what I read, not from personal experience. The key according to reputable experts is to prevent contact of the roots with the material.

Ironically (or serendipitously) I spent the morning helping some farmer friends to dig out a former greenhouse (trashed by hurricane Igor) and we were removing creosote rail ties that were used for the foundation. I was told they were in the ground for ten years: most were intact but some of the more exposed ones (on a side well drained but exposed to sun) were pretty well rotted and coming apart. There were lots of carpenters (sow bug, wood lice?) looking healthy and busy (they are very common here, come into my basement, eat paint off the floor and die). There doesn't seem to be a clear indication out there of how or when the toxicity of creosote is over. Just wondering.

I presume the creosoted railroad ties are the outer enclosure of the planter. If so, lining the box with polyethylene (or similar) should prevent the creosote from coming into contact with the soil and plants. BUT, if there is any drain from the bottom of the plastic liner, do not let it just drain into the box; instead, run a small drain tube through the side or bottom of the box, so that the ties do not become soaked with water (which could "float" creosote back up into the liner).

Good suggestions. I was thinking of using epdm (inner-tube material) which is commonly used in backyard ponds where fish live. I could overlap it on top of the ties and screw it into the concrete on the opposite side. The box is about 24' long. I would then need to construct a drain system and seal the ends to prevent water contact with the creosote. There would be gravel under the material for support and gravel on the outside of the ties to facilitate drainage. Could the drain system be holes punched into the material or should they be a sealed using plastic pipe?