Hi @KSB,
Thank you for your question about installing some long raised garden beds at your property.
You'll need to use H4-treated sleepers, which are treated for use in direct contact with the earth. They come in a variety of sizes and lengths, with 200x50 and 200x75 being common. There are traditional CCA treated pine sleepers, as well as micropro sleepers, which use a more friendly treatment which is great for vegetable gardens. There are also options for hardwood sleepers, composite sleepers and concrete sleepers that you should consider.
I'd suggest using sleeper posts such as the system from Ridgi. There are Joiner Posts, End Posts and Corner Posts to allow for a variety of designs. They come in a variety of heights and widths to accommodate different thicknesses of sleepers and heights of retaining walls. They are concreted to create a nice, secure retaining wall or garden bed.
No, you can't use the fence as the back of the garden bed, as fences are not designed to support the lateral weight they will experience from the soil and water that sits in it.
Yes, it is fine to have the bottom open, and it is often better to do this. It will allow the roots more room to grow, allowing your plants to grow stronger and larger than if they were restricted.
Any high-quality garden soil is fine to start with. You can make additions such as fertiliser, compost and mulch as your plants begin to grow.
You might like to check out How To Build a Raised Garden Bed With Sleepers for some guidance, and our Top 10 most popular raised garden beds and Top 10 most popular planter box projects for some inspiration.
Let me know what you think, and if you have further questions, please don't hesitate to ask.
Jacob
