Many commercial (pre-blended) potting mixes are based on a mix of peat moss, perlite, and vermiculite. This lightweight "soilless media" wicks well by itself. Some gardeners choose to substitute (or add to) a peat-based media with coconut coir instead, since it might be more sustainable. Wood fiber products are also entering the market by way of being used as part of a soilless media blend pre-bagged by manufacturers, but it's probably harder to find by itself, and we don't have much experiencing using it in container gardens yet. (Nurseries are starting to use it, so potted plants for sale might have some wood fiber in the potting mix in addition to peat moss, bark, perlite, etc.) The other reason peat moss is sometimes substituted by gardeners is because it can be hard to re-moisten if it dries out too much (it can become hydrophobic), whereas coconut coir is easier to re-wet once it dries.

If you use a type of self-watering container, where the water reservoir sits below the root mass and is separated from the soil by a mesh drain plate, then you could use cotton-based wicks (either cut from fabric or purchased as cording/rope pieces) to help absorb water from this layer and wick it into the soil above. If it's a useful reference, here is one example of a "sub-irrigated" planter style (this is a commercial site, so no endorsement is implied): https://albopepper.com/sips.php

Sand is not necessary to add and tends to make containers much heavier and harder to move; it won't wick so much as improve drainage (when it works at all...usually so much is needed to affect drainage that it's not worth using). Avoid adding any rocks/pebbles/stone, as they can interfere with drainage and take up space needed for root growth.

Determining what plants are best suited to containers depends on many factors, with root room being only one aspect. True, some perennials have more of a taproot than others, so the size of the container will play a role in how well they grow and if they tolerate growing in confined conditions. Additionally, whether or not a planter contains a mix of several species or only one plant will determine what size it should be to accommodate enough root growth to support healthy plants. Most perennials will probably need at least a 7- or 10-gallon capacity container, or about 12 to 15 or so inches in diameter, but that will depend on the plant(s) in question.

Hollyhocks can have a sizeable taproot, but the plants only tend to live a couple years (as a biennial), so their roots won't take up as much space over the long term as a mature Echinacea (some of which are also a bit short-lived) or another species.

There isn't a specific fertilizer needed, and the N-P-K (nitrogen-phosphorus-potassium) ratio doesn't need to be precise. In the ground, most plants will have all they need supplied by the soil, but since mineral soil (clay, for example) is lacking in containers because it can worsen drainage, eventually the nutrients in potting media can become scarce. A general-purpose fertilizer should suffice; the nutrient used most by plants will be nitrogen. Don't over-apply it, though, as nutrient excesses can stress roots and make plants more susceptible to damage by certain insects or diseases. Some bagged potting mixes already contain slow-release fertilizer pellets, but if you mix your own or use a brand that doesn't contain that ingredient, you can use a liquid or granular fertilizer periodically and apply it as the label instructions state. You can learn more on our Garden Fertilizer Basics web page.

How well plants flower is partly influenced by nutrition, but more so is influenced by how much sun they receive and how well they're growing overall (which relates to room for root growth and if they're getting the right amount of water). Depending on how much sun the area you wish to plant gets, some plants will be better-suited to those conditions than others.

One addition aspect of choosing a container size and matching it to the plants you want to put in it is the consideration of winter hardiness. Roots growing in the ground are well-insulated by the surrounding soil. In a container, however, the root mass is more vulnerable to temperature fluctuations, and may freeze more often or more completely than roots in the ground, and this could harm the plants, even if they are otherwise hardy. The larger the container used, the more insulation roots will have. A wicking container might not work well enough in winter to keep roots hydrated, so watering from the potting mix from the top might be needed that time of year. (If roots in a pot don't die from cold exposure, sometimes they die back from getting too dry in winter.)

Miri