In addition, rocks can provide a warmer micro-climate for the plants in your pocket bed. The rocks act as a "heat sink," absorbing heat from the sun during the day and radiating it back to the plants at night. A plant that's only hardy to zone 7 might flourish in a zone 6 pocket bed. I've got gladiolas in one of my patio pockets, and they've been blooming beautifully there for several years now, with no need to lift them in fall.
Rosemary is a perennial that's often considered fussy. The tag may say a variety is hardy to 10° below zero, but that doesn't give the whole picture. If it warms up enough that the plant starts growing and then turns cold again, the plant gets shocked. The big heat sink provided by the surrounding rocks can make all the difference. I lost rosemary plants routinely until I tried one in a pocket bed. There, the warm/cold cycles of late winter still damaged the tips but didn't kill the whole plant.
We have several boulders around the edge of the back patio. They're just the right size to sit on. I wanted to plant behind them but didn't want a big garden bed. Pocket beds were the solution, and they gave me a place to try plants that were too fussy for other spots. A lavender that had been complaining about wet feet doubled in size when I moved it to a pocket bed. Tiny sedum and thyme starts were easy to tend while they got established. My struggling rosemary grew until it dwarfed the boulder behind its pocket bed.
What if you don't have a big boulder? Try putting a pocket bed against a foundation wall, or at the base of a set of stairs. A complete circle of smaller stones can also make an excellent mini-bed. On a steep hillside, a semicircle of stones on the downhill side of a plant can work wonders. You can backfill behind the stones with a rich soil mix, and the stones will keep the soil from eroding away from the plant.
Here's how to construct your own pocket bed.
Gather some large rocks. You could also buy bricks or "castle rock" concrete pieces, but I love the look of natural stone, and it was free for the taking from the rock pile at a local construction site. Dig out an area next to the rock, removing any sod or weeds. Fit a first layer of rocks along the edge of your new pocket bed, digging a little to get them well seated. I didn't bother putting sand or pea gravel under the rocks, but I did pack a little clay here and there to firm them into place.
If you're using large rocks, say the size of bread loaves, then you may only need a single curved row. Look at the ends of the rocks as you line them up; some will fit together better than others. It's a little like putting together a jigsaw puzzle, but precision isn't required. Gaps between the rocks will help with drainage. If you've got an especially big gap, plug it up with a little stone. If you're using smaller rocks, you'll need to stack another layer or two of rocks on top of your base row. Juggle them around a bit to find the best fit, and they'll stay put without mortar. When you've built up the rim enough for a bed that's at least 8 inches deep, you're ready for the next step.
With a full sized raised bed, you'd probably double dig and amend the soil inside the bed. Pocket beds are small enough to fill with nice potting mix instead, without breaking the bank. Choose a good soil-less mix so your pocket bed will have excellent drainage. A bottom layer of coarse material like gravel can ensure good drainage even if your soil mix settles a bit. Add compost or time release fertilizer to provide nutrients. Polymer moisture crystals are a great way to improve water retention without causing soggy soil.
Now your pocket bed is ready for plants! Think "thriller, filler, and spiller" when choosing plants to fill your new bed. Plan your planting as you would for a container, so your bed becomes a beautiful accent in your landscape. Alpine and rock garden plants will appreciate the excellent drainage. For a plant that needs little water once established, consider sedums, with a variety of species ranging from low growing groundcovers to tall, dramatic cultivars.
If your pocket bed is near a doorway or a seating area, be sure to include some herbs and other fragrant plants to enjoy. Herbs such as lavender and rosemary love growing in pocket beds. Tuck thymes into little nooks where they can cascade over the edge of the bed. Fill in spaces with annual flowers or tender herbs like basil. I love sitting on my patio rock, enjoying the sweet scents of 'Rose Petal' Thyme and blooming Chocolate Daisy (Berlandiera lyrata).
Pocket beds provide all the advantages of growing in the ground as well as those of container cultivation. Do you long to grow fussy plants that demand a little more warmth or better drainage than your yard can provide? Pocket beds may be the answer!
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For additional information on providing excellent drainage in raised beds, see PalmBob's article on building raised beds for succulents.
Photos by Jill M. Nicolaus.