I am often asked about how to fill window boxes. People are concerned about weight more than anything else but also how long the box will hold water and what plants can grow well in them. For this there are several things to consider like what filter material to use, and how to keep your flower box watered, but the most frequently asked question, is what soil to use.
For your soil selection I recommend a potting mix as opposed to a potting or garden soil as it is going to be much lighter weight and have better moisture retention. Potting mix is actually a soil-less medium that is great for planting a variety of plants especially with limited space. Whether you want to grow annual flowers, herbs, or other small food plants they are going to be exposed to sun, rain, and wind and they need a good foundation and that all comes down to soil. Regardless of what you intend to plant you are going to want some sort of filter layer at the bottom of your window box so excess water in your box that drains out the bottom won’t carry your soil away with it. A single layer of landscape fabric or about one quarter to one half inch layer of pea gravel will help maintain your soil levels so you are not constantly replacing it.
Another great addition to your window box filler is vermiculite or perlite both are naturally occurring minerals that are mined and processed so which one to use depends on what you are growing and your solid needs. To make is short and simple, vermiculite is better at moisture and nutrient retention, and perlite is better at draining and releasing humidity; so if your plants are water loving use vermiculite and if your plants prefer a drier soil, like cacti and other succulents, then use perlite. There are as many opinions on the exact ratio to use as there are articles about which ratio to use, but they will all agree that either will improve aeration and plant health when used in the right situation, and since neither decays it is a good investment to maintain the health of your soil for several growing seasons.
Sometimes, for larger boxes, a filler can be chosen to reduce weight. A good way to do this is by using leftover styrofoam from an online order. Styrofoam cannot be recycled through normal recycling centers and has to be taken to specialized manufacturers for reprocessing. Using it as a filler in a window box is a great way to keep it out of landfills which helps the environment and also helps reduce the overall weight of your box.