How to Build a Rooftop Garden

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How to Build a Rooftop Garden

Although the history of roofgardening goes back to the Hanging Gardens ofBabylon, it was not until the advent of themodern high rise apartment building that thispeculiarly urban field of gardening came into itsown. Today, in spite of the many problems thatbeset them, thousands of roof-gardeners havediscovered that an upstairs garden can help tomake urban living very tolerable, whether onspacious penthouses or on tiny balcony terraces.

A realistic approach, and an understanding ofthe difficulties that must be faced are primerequisites for a successful roof garden. Since theroof gardener starts with nothing but a barefloor his first concern is something to hold thegrowing medium. The containers may rangefrom standard-size window boxes to built-inmasonry beds. The former, being only 6 or 7 in.deep, play a limited role in roof-garden land-scaping, because few plants will winter over ifsthem—which means that they must be replantedeach spring in the traditional fashion. Thenearest thing to natural growing condirions andnatural landscaping is achieved by the use ofmasonry-retaining walls, 18-24 in. high. Suchraised beds have the greatest potentials of all,but arc usually practical only on the terraces ofcooperative penthouse apartments. The greatmajority of roof gardeners depend on terrace-boxes and tubs to contain the soil and supply thegarden frame-work.

Terrace boxes may be anywhere from so to18 in. deep and wide-4- or 5-ft. lengths aremost practical. The best all-round material iswood, especially for the build-it-yourselfgardener. Redwood and cedar are best, but firand pine serve very well if treated with a preservative. Those who want a classic lookmay prefer concrete, or the modern look ofreinforced fiberglass. Both offer excellentdurability and insulation but at a higher cost.

Perhaps "planter" is a better word to use thantub, for in this category is included not only thetubs and barrels but wooden cubes, concreteurns, fiberglass cylinders, large terra cotta potsetc. Fine accent effects are possible when thecontainer that will bring out the best in a plantis chosen or a plant to show off the container atits best.

Whatever type of planters or beds are used,the soil-mix that fills them must be of thehighest quality. Suggested mixtures per cu. ft.are as follows:

Preparing such soil mixtures is a tediouschore, but one that pays handsome dividendsfor years to come. To an even greater extentthan in a conventional garden, soil is the key tosuccess. When unimproved soil is dumped intoroof-garden planters it becomes muddy whenwet and rock-hard when dry—and since itrequires frequent watering, there is an excessiveleaching-out of nutrients. After a couple ofseasons of this, very few plants will survive.

Although some leaching is inevitable it canbe kept to a minimum by proper attention todrainage. An inch or so of drainage materialshould line the bottom of the planter. Shards orclinkers seem to work out best, especially theformer, which can be placed over drainage holeswith the concave side down. If a layer ofsphagnum moss or fiberglass is then laid over thedrainage material the result is a slow, steadydrain-off of moisture, with a minimum ofleaching. "Outside" drainage is providedsimply by raising all planters an inch or two offthe floor. Masonry beds must have built-in"weep-holes" every few feet.

A year-round surface mulch is of greatbenefit to roof-grown plants. Mulching helps tokeep roots cool and moist under the blisteringheat that roof-garden plants must withstand inmidsummer. In winter it keeps the soil evenlycold and prevents heaving, which can other-wise be disastrous in narrow planters.

Although space does not permit a detaileddiscussion here of landscape construction in theroof garden, mention should be made of fencing and overhead protection. Before doingany planting, it is important to erect some sort ofwindbreak if none exists—for high winds canseriously damage many plants, especially whennewly planted, and the higher the floor, thehigher the winds will be. An awning, arbor orlathe house is almost a necessity, to filter the fullglare of the sun. A small pool fitted with acirculating pump can also help to beat the heat,and provide a perfect setting for plants aswell.

Trees

Planted in large containers or masonry bedswith a soil depth of 18-36 in., small trees can begrown for a good many years. For best results, choose nursery-grown, balled and burlappedspecimens not over 6 or 7 ft. tall. Those withcompact growth habits and good tolerance forwind and smog arc of course preferred. Recom-mended small trees include Honey-locust(Gleditsia triacanthos), Russian-olive (EleagnusangustifOlia), Hawthorn (Crataegas), CornelianCherry (Corpus nuts), crab apples, and SilkTree (A Ibisia brissin). Good shade trees for aspacious terrace include London Plane (Platanusacerifolia), Littleleaf European Linden (Tiliacordata) and Ginkgo (C-. biloba). For a quickaccent display Weeping \Annoy,' (Salix baby-lanica or S. vitellina) can be tub-grown ifwatered faithfully. Another fast grower,Lombardy Poplar (Populus nigra 'Italica') canbe useful as a screen.

Evergreens

Conifers for roof planting should be tightlyballed or container-grown specimens. Can-grown plants are a particular boon because theyare easy to transport, easy to fit into narrowplanters, and easy to transplant with a minimumof shock. Those most adaptable to roof gardensare the ones that thrive, in nature, on windswepthills. Pines, especially Japanese Black Pine(Pines thunhergii), head the list—followed bySpruce (Picea), Red-cedar (Juniperus virginiana).Yew (Taxus) does well with the aid of anadequate windbreak. For roof rockeries DwarfMugo Pine (P. nzugo maga) and the prostratejunipers are excellent.

You need considerabie shelter to do well in a roof garden.Heath (Erica) and Heather ((..:allutia vulgaris),however, can be used to good advantage inexposed positions if protected by evergreenboughs in winter. Andromeda (Pieris) isremarkably tolerant of roof conditions, andother good bets are Japanese Holly (ilex crenata)and American Holly (I. opaca). Rhododendronsand azaleas need considerable pampering, andare most at home in a protected masonry bed.English Ivy (Hedera helix) is a dependable ever-green vine if given winter protection, but for ahot southern exposure Winter Creeper (Euony-mus fortune/ "Vegetus") is better suited. Specialattention to the soil mixture is needed for allroof-garden evergreens (see suggested acidmixture above) and a year-round mulch is in-dispensable.

Shrubs

Almost any deciduous shrub can thrive in aroof garden if given a foot or more of soildepth, as in a terrace box or 16-18 in. tub.Some of the best choices are Abelia, WingedEuonymus (E. alalus), Butterfly Bush (Buddleia),Flowering Almond (Primus triloba), Forsythia,bush honeysuckles, Weigela. Privet can providea living windbreak, but will choke out otherplants.

Vines

Most hardy vines arc easily grown on theroof garden. Tried and true arc grapes, BostonIvy, honeysuckles, Silver lace Vine (Polygonumaubertii) and wisterias. Where possible thesevines should, like privet, be grown in separatecontainers to prevent their roots from crowdingout other plants.

Roses

The most tolerant roses are climbers, flori-bundas, and polyanthas, all of which do well in12-18 in. of good soil depth if mulching andwinter protection are adequate. Miniatureroses are fine for window box culture if winteredindoors. Container-grown plants are better ableto cope with the conditions than bare-root ones,and spring planting is best. Filtered shade isusually needed if exposure is south or west.

Perennials

Quite logically, those perennials that originallygrew in the open fields (like the daisies), or ondry slopes (like the sedums), are among thebest choices for roof gardens. The list includesEvergreen Candytuft (Iberis), Creeping Charlie(Lysimachia nummularia), chrysanthemums, Di-anthus, Gaillardia, Showy Stonecrop (Sedumspectabile), Moss Pink (Phlox subulata), creep-ing sedums, Creeping Thyme, Yarrow (Achillea) and Yucca, as well as most perennial herbs.Soil heaving is a serious menace where volumeof soil is so small, which means that a wintermulch for perennials takes on added importance.Spring planting is usually best.

Summer Bulbs

Roof gardeners who find that in such smallplanting areas perennials do not provide longenough blooming seasons can easily fill in gapsby interplanting with summer bulbs. There isone to suit almost any situation. Tuberoses,dwarf gladioli, or dwarf hybrid dahlias are madeto order for terrace boxes. Cannas arc effectivein tubs, and climbing Gloriosa-lily is a show-y,low-growing vine for trellis or arbor. Under thearbor is the best place for caladiums. Even forwindow boxes there are small bulbs likeOxalis or Zephyr-lily.

Spring Bulbs

A perfect time to improve used-up soil andplant spring-flowering bulbs is when deadannuals are removed in the fall. A spectacular,early season color display is possible, even inshallow planters, in which small bulbs can begrown. Preference logically should lean towardearly-flowering, low-growing types, but anyspring bulb will thrive in a roof garden whenplenty of sun and good drainage are easilyavailable. Unless they have masonry beds, mostroof gardeners find that the most realisticcultural approach is to plant new bulbs eachyear.

Annuals

Recommended annuals for the foregroundinclude Alyssum, Annual Candytuft, ivyGeranium, Balcony Petunia, Portufaca, Lan-tana and Verbena. For the middle ground:Chinese Forget-me-not (Cynoglossum), Coreop-sis, Ice Plant (Mesembryanthemum), FrenchMarigold, California Poppy, Snapdragon andDwarf Zinnia. For the background: AfricanMarigold, Cleome, Tithonia and Giant Zinnia.Among annual vines Black-eyed Susan Vine(Thunbergia) and Cardinal Climber are out-standing for boxes. Morning Glory pays off bestif interplanted with Moon-flower, since thelatter contributes better foliage and eveningbloom. Castor Bean is useful where a quick-growing specimen plant is needed for a tub.Cherry tomatoes and many other small vege-tables are often grown in boxes with goodresults. For those who want to be adventurousand get away from the standard bedding plantsa small cold frame is a very worthwhile in-vestment.

The singie factor in roof-garden is maintenance. In summer heat the transpiration of moisturebecomes extreme, and, since there arc no under-ground reserves, drying out can cause seriouslosses. Rain can never be completely dependedupon, since the foliage may deflect most of itfrom the soil itself. Sprinkling, except to washsoot and dust from the leaves, does more harmthan good, for light watering brings the roots tothe surface. The soil, therefore, must be given adeep and thorough soaking, with a waterbubbler or soaker-hose rather than a nozzle.Watering must be started early in the season andcontinued late.

With frequent watering comes an inevitableleaching out of nutrients, so fertilizer boosters inliquid form take on special significance duringthe growing season.

Insect pests and diseases pose a comparativelyminor problem in such limited areas. Spotspraying with aerosol bombs is usually all that isneeded, if a vigilant lookout is maintained tocatch trouble before it gets out of hand. Thechief menace is from spider-mites, whichthrive under hot dry roof conditions. On theother hand 3apanese beetles, which breed in sod,are seldom seen.

As mentioned above, good winter protectionas well as mulching is of prime importance inroof-garden maintenance. In protected lower-floor terraces an anti-dessicant spray maysurTlee for evergreens, but zo stories up aprotective screening of evergreen boughs is calledfor. Mounding soil around roses is usuallyimpractical, but collars filled with a perlitepeat mixture work out very well.

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