If your containers do not have overflow holes then I would certainly recomend drilling one yourself. You need only one at the level you do not wish the water to go above.

I had the same problem last year with my Gardener's Supply planters and have been thinking about doing this - after the week of extremely heavy rains we got in June, the water did not drain efficiently and the soil was waterlogged. Unfortunately I travel a lot and have to use self-watering containers if I want to grow anything. Have you drilled the holes in the "peaks" in the reservoir that Gardener's recommends? I believe that those are the reservoir overflow holes they refer to. I was planning on enlarging those as well.

I recently purchased several GS boxes and noted they differ from Earth Boxes in that there is no overflow hole.

However, there are 4 peaks inside the container which the instructions say to use pliers on to pull off the tips. This creates a 1/4" hole in each of the 4 peaks.

As I haven't used these boxes yet I can't say, but it sure seems like there should not be an excess water problem with these four holes.

One suggestion (other than ensuring there is an overflow hole) is to amend the planting mix with perlite, turface, grit or something else that provides for aeration. Because this is a self watering container using a fast draining mix isn't as problematic as it is in a regular container for those of us who are watering challenged ;-)

I did a quick and dirty test with an Earthbox filled with 100% Turface and found the volume of Turface was able to hold about 3 gallons of water in addition to the water in the reservoir. In a GS box that would work out to 7-8 gallons of water total in the container filled just with Turface. In an EB it would be around 5 gallons of water (smaller reservoir)

sky garden

All the homemade earthboxes have the overflow hole drilled just at the top of the reservoir, but below the soil level. That way, the rainwater can drain into the reservoir til kingdom come, but the reservoir water itself can't rise up into the soil above it, which seems to be the problem. The overflow holes at the top of the reservoir will take care of the extra water, and your plant roots can still draw water up from the reservoir.

sky garden

A question. Do your windowboxes have no overflow holes at all at the top of the reservoir? I took a look at the site (below), and I sure don't see any in that cutaway graphic.

If so, you really do need them. How strange that they omitted this vital piece of self-watering containers.

Here is a link that might be useful: Self-Watering Windowboxes

I will be using self-watering containers for the first time this year and based on all my reading drainage holes at the proper location should be sufficient. If for some reason you think the soil will take on too much water in heavy rains why not put a plug on the watering hole.

I have several of Gardener's large self watering planters & LOVE them. Very lucky people get them from me as gifts! I never heard of drilling holes in the "peaks" and my planters have never become waterlogged-the excess water from strong rain has drained out of the fill holes, but if the window boxes don't have these, I can see where it could be a problem. Have you tried contacting Gardener's Supply & asking them? Just this week I had a question about one of their products & e-mailed them. They responded very promptly & even offered me an additional discount on the product. (I probably shouldn't have mentioned that!) What I meant to say is that I've found them to be very helpful!

What is not documented well is that you shouldn't fill the reservoir until the plants are larger. If I plant my boxes in mid May, I'm not filling the reservoir until end of June.

I've seen the same problem with earthboxes. I have had tomato plants mold because they were too wet, even with the drainage holes.

Kes what type of mix are you using, it sounds like it's not draining properly.