Lilies on your windowsill? Tomatoes on your rooftop? Green thumbs aren't limited to people with back yards. Even something as simple as sand in a tray, raked into a pattern, can be called a garden (and its owner a Zen master).

Despite the challenge of limited space, apartment gardening is all about being creative and working within your space. Here are a few ideas for transforming your loft, studio, stoop, or rooftop into an urban Eden.

Rooftops and Balconies

If you have the luxury of rooftop access at your residence, you can easily transform this area into a thriving green space, with a little imagination and nurture from Mother Nature. Most rooftops are notoriously windy, so you'll need to plan ahead to shelter your plants from the elements. A glass shield provides a permanent wind shelter that doesn't reduce light and doubles as a lovely coffee-break area. In contrast, installing a netting material allows you to integrate additional vines' from beans and peas to ivies into your hideaway, while still sheltering your main plants from the breeze.

When considering a rooftop or balcony garden, it's also important to map out which areas are sunniest and which are shadiest. Plan your plant layout based on the plants' shade and sunlight needs: This will help them stay vibrant and alive.

Window Boxes

Window boxes are one of the easiest and most versatile growing tools available to the urban gardener. They can be used for salad vegetables such as leaf lettuces and radishes, herbs such as chives, thyme, parsley, sage and marjoram, or flowers such as daisies, impatiens and marigolds. Best of all, they can grow flowers and vegetables at the same time - and look beautiful.

When designing a window box, select plants with a variety of growth habits: those that grow upright, those with bushy tendencies, and those that vine or trail. If you're making a flower bed, choose plants that bloom in complimentary hues; however, also consider the shapes and colors of the foliage for the full effect. Place the tallest plants in the back of the box, a few on the sides, and let your favorite trailing plants dangle their feet over the front of the box. Be sure to thin out your stragglers (by one-third) in mid-summer so healthy plants keep blooming in full force.

Hanging Baskets

Hanging baskets are among the easiest plants to care for - a perfect choice for the fly-by-night type of urban gardener. Two of the easiest hanging plants to grow are pothos vines and philodendrons, which come in a number of varieties. Best of all, both of pothos and philodendrons can thrive in relatively dark living spaces.
Some other hanging plants for trend-savvy indoor gardeners include prayer plants, whose leaves close at night, passion flowers, which produce a stunning purple and green bloom, and chenille plants, which have fuzzy red feather-duster-like flowers.

Air Plants

If the thought of hanging baskets don't excite you, a unique and container-less option is the air plant. Air plants are Bromeliads that get their name from the fact that they grow not in soil, but instead attached to branches and rocks. Their roots serve only to give the plants a firm anchor to whatever they are growing on. All of their moisture and nutrition is absorbed through their specialized leaves.

Air plant leaves possess tiny scales, properly called trichomes, that serve two major functions. First, they assist the plant in absorbing water and nutrients by holding greater amounts of water against the leaf surface for a longer period of time. Second, they help to reflect the intense sunlight off of the leaf surface that can be so common in their growing environment. These trichomes are what give many of the air plants their characteristic gray color.

It is often easy to determine the growing requirements a given plant needs by the appearance of the plant itself. Those that have a dense covering of scales on their leaves are most probably from an area with bright light and little water. Whereas a plant with more glossy leaves is most likely from an area of lower light and higher humidity.

Plant Maintenance

Keep in mind, the most important thing for a beginning gardener - no matter how trend-savvy - is to water the plants regularly. When you forget to water, bugs will attack.

However, over watering can be just as harmful to a plant as under watering. So how can the "green" green thumb tell if his or her new plant is drinking enough?

Stick your finger in the dirt. If it's moist, you're in the clear. If it's wet, it needs to dry out. It it's dry, the plant needs a drink. In the case of the air plant, just mist it with water once a week or so (individual species will vary, of course).