Transplanting bamboos and Potting Mix

I've mentioned before that I grow a lot of different species and varieties of bamboo (about 50). Many of them are in pots, and since most of these are running bamboos which spread via rhizomes (underground stems), they can quickly fill up their pots.

Here are a few that need to be moved into bigger pots. How do you know when they're ready for a new pot?

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It's fairly obvious with some:

You'll see the rhizomes poking out of the drainage holes.

Sometimes they won't poke out but you'll still see them circling around the pot. In either case it's probably time for a bigger home!

I'm going to put these into pots that are a couple of inches bigger. I could probably go with an even bigger size, but big pots get heavy, they need a lot of potting mix to fill them, and I've heard from other bamboo growers that too little plant in too big a pot isn't ideal -- the potting mix tends to remain too wet, which will rot the roots and rhizomes of most bamboos.

This is Pseudosasa japonica, also called "Arrow Bamboo". It's marginally hardy in my area, so I'm container growing it until it's nice and big. It makes a great deck or patio plant I think.

Here's another smaller one of the same species that's putting on a lot of growth this year:

Here's a different species: Semiarundinaria yashadake 'Kimmei'. It's got a great name and produces an amazing amount of rhizomes. This was just a tiny plant last year when I received it. It's also marginally hardy here so may remain a container plant for quite a while.

Even though these aren't huge pots, they still require a lot of potting mix, so time to make some! I typically use my own potting mix because it's cheaper than buying pre-made bagged mix, but I do use the bagged mix too.

I start with 1.5 or 2 parts compost:

That's "leaf and stick mold", also called "black gold" at the nurseries around here. It's pretty much what I'd get out of my own compost pile if I had a bigger pile.

Next I add 1 part peat moss:

Next year I may try coir (coconut husk fiber) instead of peat, but peat is still much easier to get than coir, and is cheaper too.

And 1/2 part perlite to keep the mix loose:

That's the basic potting mix that I'll use for perennials, annuals, etc.

Looks good, right?

For bamboo I'll add a bit of bagged topsoil, the cheapest stuff I can find:

I'll mix it in well of course. I don't add any fertilizer until I'm ready to use the mix, because different plants will need different types. For bamboo I use organic lawn fertilizer. Milorganite is my current favorite:

It's relatively cheap, easy to find, and smells really good (to me at least). I don't have to worry about using too much or burning the plant roots with it either.

Ok, now that I have enough potting mix I can finish potting those bamboos. Pulling them out of the pots you can see that they really do need some more room:

Look at all of those rhizomes!

As I said before I'll put them into slightly larger pots:

Fill with new potting mix (with the fertilizer because bamboo is a heavy feeder), and we're done -- for now. I may need to put these into even larger pots later this summer. They grow FAST!