Plant by Numbers goes into much more detail with plant lists, substitutions, design tips and care guidelines, I'll get you started by showing you how I made the combo pictured here.
Since the Books-A-Million (where I also happen to work) was nice enough to let me give a demonstration along with my book signing, I've decided to share some pictures of the event to illustrate this post!
Step One: Choose Plants and Container
Choosing the plants is the fun part, but it's also the most important. Since they'll be occupying the same pot and getting the same amount of water and light, it's important to choose plants that can thrive under the same conditions. Of course, they should also look good together!
I started out by choosing a red Aglaonema plant at the nursery, but needed to find other plants that could take the same conditions, yet wouldn't visually compete. To better show off the big and bold leaves of the Aglaonema, I chose feathery leaved ming aralias and spikemosses (Selaginella species); two plants that also like bright light and moist soil. For a natural and full arrangement, I used a variety of plant sizes.
The pot was chosen for its muted blue-green color, since it pairs nicely with the deep green leaves of the ming aralia. It's also roughly half the height of the plants and just wide enough to contain all of the plants.
The biggest thing to look for in a container, however, is a drainage hole. A hole in the bottom of the pot is crucial since it lets water drain away rather than collecting, stagnating and rotting the roots. Along with well chosen plants, a drainage hole is what turns a mere dish garden into a long-lasting indoor garden.
Step Two: Add Potting Mix
Regular potting soil will work for most plants, but specialty mixes and amendments can be used for plants that have special needs. For example; succulents do best when grown in cactus mix or regular potting mix with a fast draining amendment like vermiculite mixed in, while moisture-loving selaginellas might benefit from the addition of spongy peat or coir to hold onto that water. But again, regular potting mix will work just fine in most cases.
Add just enough potting mix to leave room for the plants. You can always add more or remove some as you plant, but be sure to press the soil down firmly with your hands. This ensures that the soil is tightly packed and free of air pockets, which would dry out the roots or cause the plants to sink.
Sidenote: I used Miracle-Gro in the demonstration pictured above because I forgot my potting mix and had to run to Target at the last minute, but my brand of choice thus far has been Black Gold. They provided a pallet worth of different mixes and amendments so that I could make the combinations in the book, and they have worked very well for me over the last two years.
Step Three: Arrange Houseplants
Now set the largest plants on top of the potting mix so that the crowns (where the plant meets the soil) sit just below the lip of the pot. If they're too low, add more potting mix. If they sit too high, remove some. If the roots are tightly bound together, gently tease them apart before planting. This will help the roots branch out and get established, making them more tolerant of missed waterings.
Outdoor container combinations usually have a tall plant in the middle, but indoor combos do best with tall plants at the back of the arrangement. This allows each plant to get sunlight without having to compete with each other. By placing the tall ming aralias in the back, I've also given the design more depth and interest.
As you add the smaller plants, add enough potting mix for their crowns to sit at the same level as those of the larger plants. Planting each plant at the same level will help them all get watered evenly.
Step Four: Pack in Potting Mix, then Add Water
This last part is important. Once the plants are all arranged as desired, tightly pack in more potting mix around the roots so that they don't dry out between waterings. You can either continue using a trowel, or just use your hands as I did during the demonstration. When the potting mix is flat and level between the plants, gently water the whole arrangement. If the potting mix sinks between the plants, just add more and water again.
Step Five: Pat yourself on the Back
You're done! This arrangement needs nothing more than bright indirect light, moist soil and average to high humidity. If humidity is hard to provide in your home, replace the spikemosses with a low growing houseplant that doesn't require as much humidity, such as Saxifraga or Philodendron.
To find lists of compatible houseplants, loads of tips and 50 indoor container combinations that I made myself, check out my book, Plant by Numbers. It has everything you need to know for long lasting and beautiful indoor arrangements.