It depends .....

From another post: "How large a container needs to be, or CAN be, and how much soil a planting needs/will tolerate before drainage & saturation is an issue depends on the 3-way relationship between plant mass, container size, and soil type. We often concern ourselves with "over-potting" (using a container that is too large), but "over-potting" is a term that arises from a lack of basic knowledge about the 3-way relationship noted, which should logically determine appropriate container size(s).
It's often parroted that you should only move up one size in containers when "potting-up". The reasoning is the soil will remain wet too long and cause root rot issues, but it is the size/mass of the material and soil type/composition that determines both the upper & lower limits of appropriate container size - not consecutive volume progression.

Plants grown in slow soils need to be grown in containers with smaller soil volumes so that the plant can use water quickly, allowing air to return to the soil. This (smaller soil volumes) and the root constriction that accompanies it will cause plants to extend and gain o/a mass much more slowly - a bane if rapid growth is the goal - a boon if growth restriction and a compact plant are what you have you sights set on.

Conversely, rampant growth can be had by growing in very large containers and in very fast soils where frequent watering and fertilizing is required - so it's not that plants rebel at being potted into very large containers or volumes of soil per se, but rather, they rebel at being potted into very large containers with a large volume of soil that is too slow and water-retentive.

We know that there is an inverse relationship between soil particle size and the height of the PWT in containers. As particle size increases, the height of the PWT decreases, until at about a particle size of just under 1/8 inch, soils will no longer hold perched water. If there is no perched water, the soil is ALWAYS well aerated, even when the soil is at container capacity (saturated).

So, if you aim for a soil composed primarily of particles >1/16", there is no upper limit to container size. The lower size limit will be determined by the soil volume's ability to furnish water enough to sustain the plant between irrigations." You can grow a very small plant in a very large container if the soil holds little or no perched water (saturated soil).

Pine shavings or small pieces of any wood product other than conifer bark are not such a good idea on the bottom of containers. They break down quickly & tend to cause varying degrees of N immobilization, which increases as particle size decreases. If you want a wood product on the bottom of the container to 'take up space', large pine bark landscape mulch is best.

Don't use these products, packing peanuts, soda bottles, bark, wood chips, or other, thinking that it is an aid to drainage - it is not. The practice simply raises the level of saturated soil in the container - so its value lies wholly in the fact that it decreases the volume of soil it takes to fill a container and can lighten the container's o/a weight.

Al

Hi Al,

To 'take up container space',I may get the large pine bark landscape mulch. Will this mulch breakdown over 2 yrs or so and clog up or will it retain its shape?

Since you know a lot lemme ask you a question. I live in s.e.PA. To keep these perenials(Monardas,penstemons ) in pots, in a deck(which is on 2nd floor), what kind of pot(big,wide,insulated with 2 layers etc) would you recommend, so that they survive winters?

Tks...Kris

. . . will this mulch breakdown over 2 yrs or so and clog up or will it retain its shape?

You should be ok. There will be some decomposition, but I'm quite sure the pieces will remain largely intact. The larger the chunks the slower they will decompose.

To keep these perennials . . . in pots, . . . what kind of pot (big, wide, insulated with 2 layers etc.) would you recommend, so that they survive winters?

See the link below, You asked that question already in another post & I answered it.

Group your containers against the outside wall of your building & then insulate around the outside perimeter of the containers with a tarp or anything else imaginative you come up with. The object is to NOT insulate between the plants & the outside wall, but to allow as much heat to pass through the wall as possible. Then, take measures to help TRAP/CONTAIN that heat so it helps to warm your plants.

Al

Here is a link that might be useful: Hardiness