Joanne - Adequate/good drainage doesn't depend on what you add to the pot, it depends on the soil you start with. Contrary to popular belief, even adding perlite to peat/compost/coir-based soils doesn't improve drainage. It simply reduces the volume of water the soil will hold per any given volume.

Neither do "drainage layers" improve drainage. They simply reduce the amount of soil in the container, thus reducing the total volume of water in the container at container capacity (the state of water retention after the soil has been completely saturated & has just stopped draining). The "better than rocks" material does nothing beyond taking up space in the same manner that a soda container would, so please save a buck - choose a less expensive way to reduce soil volumes.

You'll probably find enough information in the link I'll leave to get you headed in the direction of a solution to your water retention issues.

Good luck.

Al

Here is a link that might be useful: More about water relationships

Al, thank you for that link.... it is probably more info than I was looking for! :) I'm new at all of this, and need to keep things pretty simple at this point.

If it is only about the soil, then why are my terra cotta pots thriving (same soil, same plant, same location) while my window boxes are not... I water the window boxes less, yet they never seem to dry out. Just looking for a simple solution to help plastic act more like a porous material like clay.

Plastic will never be terracotta, not even with holes drilled and a wick added.

Josh

The terra cotta walls and bottom are gas permeable, which means they allow water vapor to pass through the walls. This dries the soil faster and helps air return, which helps to reduce the negative impact of to heavy soil that doesn't want to drain. It also allows foil, growth inhibiting gasses like methane, CO2, and sulfurous compounds that build up under anoxic/anaerobic conditions (soggy soil) to escape through the container walls.

Please don't take what I say as argumentative. I WANT you to do well, so I'm still going to encourage you to try to understand the principle of perched water in containers and how it is affected by soil particle size. Even if you only use the info to help you deal with heavy soils, it's better to have knowledge on your side. It only LOOKS daunting. Most people get the concept right away. It took me a long time before I could say this because it sounds like I'm boasting, but I'm really not: Literally hundreds of people, both newcomers to container gardening and those that have been gardening for a very long time, have said in so many words that "Now I finally get it. Now I understand why I was having so much trouble growing in the soils I was using ....." and so on.

One way or another, you'll have to deal with the fallout when using heavy (water retentive) soils. I, or others here, can help you avoid the issues by helping you build a soil that better suits your application, or by offering tips that can at least partially reduce the negatives you're dealing with.

Drainage layers or gimmicks aren't going help, but tilting your containers after you water and using a wick can help considerably - it's in the link I offered.

Good luck.

Al

tapla what are your thoughts on those self watering containers? Using your 5:1:1 mix growing peppers in hot Florida sun, would you refrain from using these containers? I'm just thinking out loud at this point.

I don't use SWCs, but if I did, I wouldn't refrain from using them because of the heat; I'd simply find ways to help reduce root temperatures.

Apologies to Joanne for straying a little from the OT.

Al

Regarding the original question, you can fill up space in large containers with soda bottles, empty flowerpots, Styrofoam peanuts, etc. For annual flowering plants, I only use the top 1/3rd of the pot for soil. Never heard of the 'rocks' you mentioned.

What are your growing conditions? Full sun, shade?

Jane

I fail to see the benefit of changing to deeper containers and then adding spacers such as rocks or soda cans to reduce the increased volume. If reduced volume was the goal why would you change to deeper containers? Al

"How big should the drainage holes be? How many would you recommend?"

How big/long is the windowbox container? How many are already in it and what size are they? That could give you a guideline as to size. Drill several inches apart so as not to weaken plastic. Use needlepoint plastic if you want but window screening is cheaper.

No rocks or cans or anything. Use a good quality purchased soilless mix or mix your own if you are so inclined. Add time-released fertilizer or plan on fertilizing with liquids every week or two.

I like the tall containers so trailing plants can hang over. the larger containers are wide and I can stuff them with plants. I don't need depth, just width and height. Filling large containers with soil is too heavy and expensive. It's perfect for annuals.
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Another plus is they are easy to move around.

Jane

my thoughts-

There should be a drainage whole ~3/8" at the lowest point in the container-no matter how the container is tilted or oriented. so drill holes every 90-45 degrees to assure that. (If we get a torrential rain storm- I tilt pots to drain them faster)

If you think about it, a container could not hold a pot full of water more than 10 minutes with just a few holes drilled. So Holes are not the limiting factor to drainage... the mix is--in my opinion.

If their were 6 holes and none were at the lowest point, even the fastest draining mix would still be wet at the low point.

On container size-- I do not have enough resources to always have the exact right size pot for every situation.
So if I need a 7 gallon & the closest I can find is A 10 gallon, Old plastic water bottles are put in the bottom to shrink the 10 gallon down a bit. This might make the pot a little top heavy, so some rocks get put in the very bottom. ( I could put water in the water bottles now that I think about it)