Well, I can't say for sure what they used in those particular planters, but I can tell you what I do. I use potting soil, usually miracle gro mix. If its a really deep container, i line the bottom with old pots, like 4 or six inch plastic pots, to take up some of the space and allow for some drainage. I always reuse the soil for a few years and they always turn out beautiful. The key is to keep them happy, evenly watered and fertilized. Good Luck!
Sarah

Yes,the advice given from Sarah is very good.You'll find that any potting soil will work for you,and using a time release fertilizer is a great idea.It'll feed for about 4 to 6 months and is triggered when you water.Cram your containers to the max for best results....it makes a better display and forces the plants to cascade over the sides of the pots.If using Petunias,make sure you spray bt
around the 1st week of June to avoid budworms.
Have Fun!

Correct me if I am wrong. Here is how I am visualizing it.

Each wooden planter will contain many small containers that are close together.

Or are you saying to fill each wooden planter with the potting soil?

Thanks!

What they're saying is - if you're working with large planters, instead of filling it up entirely with potting soil/potting mix, line the bottom with old flower pots (upside down). This takes up some space in the container and you'll have drainage and won't be using bags and bags of potting mix.

Remember also, large planters are very heavy once filled and watered. You might have to assemble them where you want them because moving them can be a chore.

Thank you so much duluthinbloomz4! :)

For planters of such large size, Miracle-Gro or other bagged potting mixes will be, at best, a rather mediocre choice - even if you intend to change the soil annually. If you plan to use the soil for more than a year or two, the choice becomes less desirable. A substantial mineral component (needs to be much coarser than topsoil or garden soils) will help the soil retain structure/ durability/ longevity, which bagged soils lack.

There is an active discussion about "Container Soils" if you'd like to join in. Even if you're not willing to make the effort to find the ingredients to make a version of the soils discussed, you'll still come away with a good understanding of what makes container soils work. BTW - making your soils and understanding why the ingredients work well together is not only satisfying, it's also much less expensive in the long run.

Take care - best of luck.

Al