Planting window boxes properly consists of several key steps that need to be followed so that you have easy maintenance and use. You'll want to make sure you use potting mix for the best result as the loose soil helps flowers and plants take root and grow the best. It may cost a little more than potting soil, but make the investment in quality potting mix and your plants and flowers will benefit tremendously.
Drain Holes and Window Box Liners
Determine if you will need liners for your window boxes. If you have wooden window boxes then the answer to that is yes, you need them. Window box liners can significantly add to the lifespan of a wooden window box. If you have PVC, composite, or plastic window boxes then you can usually plant directly in the boxes which tend to be rot resistant or rot proof.
Even if you are not using window box liner you'll still want to line the bottom of the box for good reason. A layer of filtration on the bottom of the window box and covering up the drain holes will help trap dirt and soil in the box while allowing water to drain out the bottom. There are two methods to create filtration. The first method is to pour a 1" layer of pea gravel on the bottom of the box. This allows potting mix to be trapped above it so that water can wiggle out and escape the bottom of the box through the drain holes.
Pea gravel also helps keep roots out of contact with standing water which may temporarily develop after a heavy rainfall. For this reason you'll want to make sure that you have excellent drainage. You can drill lots of little weep holes approximately 1/4" diameter in the bottom of the box unless the manufacturer of your window box does it for you or uses an alternate method. But make sure you have good drainage regardless or puddling from rain will kill off your flowers.
The second method to creating great filtration in your window box is the use of landscape fabric, or weed block fabric. You can cut 1-2 strips of it the size of the bottom of the window box and lay it on the bottom. Cover the drain holes up so that any drainage out the bottom of the box will have to pass through this fabric first. This should help clean the dirt and separate the water so that the water coming out the bottom is 99% clear and clean. You can even do both where you would lay the fabric down first then cover with an inch of pea gravel. If you're using a liner I recommend putting the filtration in the liner and making sure that both the liner and the window box have drain holes in the same approximate location and can drain easily. A liner may need to be propped up a little if there is any chance there will be drainage problems.
Adding Soil and Self-Watering Mechanisms
The next step after you have figured out your drainage, liners, and filtration is to add soil. I highly recommend using high quality potting mix and not potting soil. Potting mix is looser and better at water retention. It's also easier for new flowers and plants to take root in. If you've already laid down your filtration then you can put your potting mix directly on top of it.
Some manufacturers offer a self-watering option. Self-watering window boxes have become very popular for people with busy lifestyles or window boxes in hard to reach areas. You can make window boxes self-watering several different ways. The easiest method is to simply tie in the sprinkler system to your window boxes. If the box is high up on the house sometimes an irrigation company can run drip lines to the box that are on a timer. If exposed drip lines are in a spot that won't be attractive then some companies make window box reservoir systems that simply slip into the window box and act as a backup watering tank to store extra water and reduce watering frequency.
Planting Flowers and Plants
For planting window boxes you'll want to consider your root space. The larger your internal planting space is then the greater soil capacity you will have and the more plants you can put in your box. An 8" tall and wide window box is usually adequate for flowers and seasonal plantings. But for deeper rooted plants I recommend a 10" window box. This will allow the roots to grow deeper making it less likely that your plants will outgrow your box.
Another advantage to having deeper window boxes is that you can layer your plants better with a front row and a back row. This allows for better combinations where hanging flowers and vines can be strategically located in the front while filler flowers and taller plants can sit in the back row.
About the Author
Matthew Buquoi is the owner of Flower Window Boxes, a window box manufacturing company specializing in self-watering window boxes.