How to plant a windowbox or,
I spent hours at Gethsemane Gardens yesterday and sat in on an urban container gardening class or,
Go into ecstasies at the color and scent and glory of the garden center. Try to remember you have a budget. Choose plants carefully according to color (either contrasting or complementary colors), size (short / medium / tall), and form (ideally, each flower type should have a different shape). I almost always prefer to plant in complementary colors -- more soothing compositions. Contrasting (such as red/yellow/blue) gives you more pop, or wow! factor. These boxes will be, from nearest to farthest, red/orange/yellow (dining room), blue/white/pink/purple (living room), and old-fashioned faded pink-green-mauve-white (Kavi's room)
Some great choices for early spring
Tall plants: stock, linaria.
Medium plants: ranunculus, nemesia, mini daffodils, hyacinth, daisies.
Short plants: pansies, violas, mini daisies.
Trailers: creeping jenny, bacopa, variegated ivy.
In Chicago, I pretty much plant annuals in my window boxes, because the odds of their surviving the winter are slim to none anyway. Annuals give you a lot of bright color for relatively low cost. One of the exceptions is hellebore -- I love it so much that I plant it in my spring window boxes anyway, even though it's a bit of a waste of a perennial plant. If I had a yard, ideally one with trees, once they were done blooming, I'd move them to under the trees, so that next spring, they could come up again on their own, one of the first blooming perennials. That far right tall flower in the photo is hellebore. It comes in shades from white to pink to purple to dark red and dark blue, often with striking green veining on the petals.
Take sun / part-sun / shade into account. For example, I adore ranunculus, but when I tried planting them in my west windowboxes the last two years, they curled up and died. The garden lady said they can't take much heat. So I'll try them in the north windowboxes this time around, which are bright light, but not so much sun/heat, and hope they do better. Each plant at the store should be clearly labelled with its light requirements. Pansies are tolerant of a variety of light/heat conditions, and geraniums (not yet arrived at my store) even more so. Good for beginners! For my hot west windows, they recommended mini daffodils for spring. (Any other suggestions? I need to fill those containers out a bit more.)
Buy good plants. They may cost half as much at Home Depot, but they'll likely also live half as long. Often, you can divide plants to help reduce cost -- for example, they sell big containers of stock at Gethsemane for $24.99, but those can be divided into about eight different plants, enough for 2-3 containers. Ask at the store for advice.
Don't forget to buy potting soil (not garden soil or other soil, but potting soil, which is light and full of container-y goodness, plus pre-fertilized a bit), slow-release fertilizer, soil moist, mulch, and any containers you need/want/crave. (Note: glazed pottery won't survive outside in a Chicago winter; plan to bring those in. I tend to plant my tropical plants in glazed containers, so I'm reminded to bring them in in the fall. (Hibiscus, jasmine, etc.)
When you get home, and have successfully talked someone into helping you lug all that stuff into the house, throw out the dead plants you meant to throw out last fall (but you were out of town, so at least this year you have a semi-decent excuse).
Remove and discard two-thirds of the dirt in the box from last year. It's old and tired. Add lovely fresh potting soil). Leave about two inches between the top of the soil and the top of the container, for mulching. Do have someone hold the baby so that she doesn't crawl out on the balcony and cover herself in dirt. (Note: this is a futile effort, because half an hour later, she will dig into the potted indoor bamboo, and cover herself with dirt.)
Curse because you meant to pick up some soil moist and forgot. Now you have to wait until tomorrow before continuing.
Rehydrate your soil moist and scatter in box. These are tiny little granules that you scatter into your windowboxes to help retain moisture. If you have a windy third-floor balcony in Chicago, retaining moisture is a big deal. The garden store lecture I went to yesterday recommended rehydrating them before adding them to the box, because otherwise, you don't know how big they're going to get (they swell a lot), and they could end up pushing your plants up in an unattractive manner.
Scatter some slow-release fertilizer in box. Toss soil to mix. The slow-release stuff claims it's good for six months, but the garden store says that by July/August, they always start fertilizing with regular stuff again. They recommend using regular fertilizer at half-strength what the box recommends, but twice as often as the box recommends. I expect I'll fertilize when I remember to, about half or a quarter as often as I intend to.
Plant with bare hands if you want to enjoy the sensuous delight of planting; with gloves if you're worried about getting poisonous fertilizer on your hands. Try to ignore parallels with condoms, etc.
Ready to plant! Take plants out of their planter pots, one by one. As you take them out, check to see if the bottom of the plant is root-bound (very tightly matted with roots). If they are, gently break up the bottom roots with your fingers, so that they can spread out a little in their lovely new container home. A little water can make this job easier; you might want to prep a bowl of water at your planting area in advance.
For a window box that you want to look nice both from inside your apartment and outside, start from the center long row with the tall plants. Then plant on the inside and outside of that row with the medium plants. Then along the long sides, with the short/trailing plants.
Plant in odd numbers -- 1, 3, 5, etc. You don't need to be absolutely strict about this, but in general, it's more naturalistic and pleasing to the eye. So a box might have three stock, one ranunculus, five pansies, two creeping jennies.
Finish planting with a nice layer of wood mulch across the soil. It a) looks nice and finished, and b) helps retain moisture.
Water thoroughly!!! Your poor plants have just been through a traumatic experience -- they don't like change, and could use a good stiff drink!
Water daily! Ideally either early in the morning or late in the evening. Watering during the sunny part of the day loses a lot of water to instant evaporation, plus, water sitting on the leaves/blossoms can actually act as a lens and burn the plant. (In Chicago, water twice daily in heat of summer.)
Enjoy! Until the heat of summer rises, when you may need to start all over again, because spring flowers probably won't make it through the summer. (My north-facing ones sometimes mostly do, but west, no chance.)