Is that a railing behind it? Maybe you could screw some hooks on the back of planter so that they hook on the railing just enough to keep it from turning over. They make different size metal plant hooks that screw to a wall to hand flower baskets on,measure the distance from railing to box and find a hook about that size. Something like this

QIEGL Hanging Plant Bracket 6inch Metal Plant Hook Shepherds Hook Decorative Wall Plant Hanger for Hanging Lanterns, Bird Feeder, Flower Basket, Home Decor Indoor & Outdoor 2 Pack (Black) https://www.amazon.com/dp/B08FB8Q57X/ref=cm_sw_r_apan_i_MXSGJ750G6VPK7RHYGQB

Drill some holes in the upper part of the planter and tie or zip tie it to the railing.

i though the same about wiring it to the railing.. then i pictured it ripping the railing off the wall .. lol ... ill have to mull this over.. maybe come up with something ... ken

Or you could empty the planter and set anchors into the brick then refill. Either way I would put wedges under the front to slightly tilt it back.

Hydrangeas can grow crazy, I think it will always be a problem at some point.

Drill some holes in the bottom and secure it into the ground with rebar?

@Kate I was thinking since it's a climbing hydrangea that's supposed to grow onto the brick, most of the plant weight won't be on the planter.

@functionthenlook I wedged in some 2x6's under it to tilt it backwards as a temporary solution. Investigating large L brackets used for kitchen countertops as a possible (ugly) solution. The problem is the planter is made of thin cedar panels (1/2 inch or so). My concern is the bracket will stay in place and it'll straight destroy the cedar. Even if that happens I think it will prevent it from falling over entirely.

The 16” is the problem. I only see two solutions:

1) turn the planter 90° so that the 33” side is against the house. But that might not work for long either

2) brackets to connect it to the brick or the railing or both.

If the Schizophragma (which is not a Hydrangea) is intended to grow on the housewall I’m puzzled as to why the planter is at right angles to the wall.

@floral_uk z.8/9 SW UK There's a large window well right in front of that wall

I see. That’s not visible in the picture. How is the Schizophragma going to reach the wall? Will it have to bridge the window well? I’m just not convinced the plant or the planter are suited to the situation.

@floral_uk z.8/9 SW UK A couple more pictures of the location. That wall with the windows is about 20 feet tall, I was hoping the false hydrangea would climb up and I’d be trimming it from around the window themselves.