Raised Bed Gardening 1

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A l A b A m A A & m A n d A u b u r n u n i v e r s i t i e s

www.aces.edu

ANR-1345

Raised bed gardening is a convenientand easy way to produce homegrown

vegetables. Unlike traditionalin-ground gardening wherelots o space is usually

required, raised bed gardening is aperect alternative or people whocannot garden due to limited

garden space, poor or rockysoil, inadequate soil drainage,or physical limitations.

Regardless o amily size or gardening experience,raised beds present homeowners the opportunityto grow vegetables themselves.

The idea o raised bed gardening is nothing new.For centuries, armers and gardeners have mound-ed soil up to grow plants. Raised bed gardening is

unique in that the soil level is higher than thesurrounding soil, 6 inches to waist-high, andenclosed with materials to prevent soil rom spill-ing out. The contained soil is ormed into plantingbeds 3 to 4 eet wide, small enough that a personcan maintain it without actually stepping onto anddisturbing the planting area.

Raised Bed

Gardening

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2Alabama Cooprativ extnion sytm

Raised bed gardens offer several advantages (andsome disadvantages) over conventional garden plots:

Advantages

Raisedbedgardenscanhelpmaximizeall

availablespaceandaretypicallysmallerthan

traditionalgardens,makingthemamore

convenientoptioninareaswithlimitedspace.

Raisedbedgardenscanbeusedasasolutionfor

areaswithpoorandrockysoilorslopedterrains.

Bedsareusuallylledwithhigh-qualitysoilmixes

thathavelargeamountsoforganicmatter,which

improvesdrainageandmayincreaseyields.

Soilraisedaboveground-leveltendstodrain

betterandwarmupmuchquickerinthespring,

thusallowingforfasterseedgerminationand

transplantgrowth.

Highersoillevelsandimprovedsoilqualityoffers

betteraccess,lessmaintenance,andeasierharvest.

Denseplantingtechniquesresultinhigher

productionpersquarefootofgardenandhelp

reduceweedseedgermination.

Raisedbedgardenscanbeenteredand

maintained soon after rains or irrigations

withoutcompactingsoils.

Noexpensivepowercultivationequipment

isneeded.

Theformalorderlinessandarrangementofa

raisedbedgardencanbeextremelyattractive

andaprizedadditiontothehomelandscape.

Disadvantages

Elevatedbedstendtodryoutmorequicklyin

thehotsummermonths,thusincreasingthe

needforsupplementalwatering.

Frameandsoilmaterialsforestablishinga

raisedbedareanaddedexpense.

Limitedrotationofcropfamiliesmayleadto increasedsoil-bornediseasepressureand

nematodeproblems.

Increasedplantdensitymayincreasesomepest

concerns,especiallyfoliardiseases.

Theyarenotwellsuitedtosprawlingvegetables

suchaswatermelons.


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Raised Bed Gardening 3

Location

As with any garden site, a raised bed should belocated in ull sun or best production. A minimumo 6 to 8 hours o direct sun is required or most

vegetables and owers. Expect less-than-optimumproduction and leggy plants i ull sun is not avail-able. The bed should also be located in a convenientlocation near the home and a water source. Regularirrigation is necessary or raised beds.

Materials and Size

Raised beds can be made just by mounding the soil,but these beds require a lot o maintenance. Mostgardeners preer to use raming materials to containthe new soil. Old railroad ties, landscape timbers,

wood planks, rock, concrete blocks, or decorativebricks are commonly used to hold and raise the soil

up 6 to 12 inches. I wood products are used, theyshould be treated with wood preservative to increasethe lie o the structure.

Materials or building a raised bed can vary greatlyrom inexpensive to expensive. Depending on therame materials and growing media used, one canspend as little as $30 or as much as $275 or a 4' 4' 12" raised bed. Construction or a 4' 8' 12"raised bed with twice the growing space can costanywhere rom $50 or a rame and soil to $515.

Table 1 lists the common rame materials used toconstruct a raised bed, the quantity o materialsneeded, and the average costs associated withbuilding our dierent-size beds.

Table 1. FrameExpensesandComparisonsforaRaisedBed*

Frame Materials Quantity

Cost for

4' 4' 12"Raised Bed

Quantity

Cost for

4' 8' 12"Raised Bed

Pressure-treated lumberplanks(2" 12" 8') or (2" 12" 12')

2 $20$25 2 $30$38

Landscape timbers(4" W 3" H 8' L)

6 $21$25 9 $32$36

Recycled plastic composite lumber(5/4" W 6" H 16' L)

2 $60$90 3 $90$230

Commercial raised bed garden kits 1 $75$170 1 $100$355

Retaining wall block and decorative stone(3" H 6" D 12" L)

54 $102$216 78 $147$312

Frame Materials Quantity

Cost for

4' 4' 8"Raised Bed

Quantity

Cost for

4' 8' 8"Raised Bed

Used railroad ties(8" W 8" H 8' L)

2 $20$27 3 $30$40

Concrete blocks(8" W 8" H 16" L) or (4" W 8" H 16" L)

14 $20$22 22 $30$35

* Prices based on spring 2009 retail data


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The size o a raised bed depends on the gardenerand can vary based on need. Ideally, rames willrange in size rom 4 4 eet to 4 12 eet. The4-oot width is preerred because it allows oreasy reach rom either side without requiring thegardener to step into the bed, keeping soil compac-tion to a minimum. The length o the bed can also

vary depending on the type o construction materialsused and available space. A bed 4 to 12 eet in lengthis suitable or most home gardens.

A soil depth o 6 to 12 inches is desirable as thiswill allow or improved drainage and adequate rootdevelopment to produce healthy plants. Beds may behigher and deeper or better access but require moresoil or a porous bottom or adequate drainage.

Concerns sometimes arise over the use o treatedlumber in vegetable beds. The most common wood

preservative traditionally used (chromated copperarsenate, or CCA) was phased out on December 31,2003, or virtually all residential use, including raisedbeds. Two other products, ACZA (ammoniacal copperzinc arsenate) and ACQ (ammoniacal copper quat)have replaced CCA and may be used or raised bedconstruction. Well-documented research has shownthat CCA, ACZA, and ACQ may be saely used toconstruct vegetable beds. However, some gardeners

still preer to line the sides o beds with polyethyleneplastic so that roots do not come into contact withthe material. Do not use plastic on the bottom o thebeds as this will prevent drainage.

Soil Preparation

One o the greatest advantages o raised bed

gardening is the ability to amend the soil or to createnew soil or the bed. This is especially important inareas where soils are high in clay and tend to drainpoorly, or on newly constructed areas where severesoil compaction has occurred. Raised beds are otenthe best option in these situations.

Many growing media options are available orcreating a raised bed. Any combination o purchasedtopsoil, compost, fne pine bark mulch or soilconditioner, and/or peat will work well or growing

vegetables in a raised bed. Commercially prepack-

aged growing mixes that contain such items are alsoavailable. Like rame materials, new growing mediacan vary greatly rom inexpensive to expensive. Table2 lists the common growing media used to constructa raised bed, compares costs by bulk and by the bag,and shows the average costs associated with buildinga 4' 8' 12" bed, or 32-cubic-oot bed.


Table 2.GrowingMediaExpensesandComparisonsforaRaisedBed*

Growing MediaBulk

(Cost per

Cubic Foot)

Cost for4' 8' 12"32-Cubic-Foot

Raised Bed

Bags(Cost per

Cubic Foot)

Cost for4' 8' 12"32-Cubic-Foot

Raised Bed

Topsoil $0.75$1.00 $24$32 $1.00$3.00 $32$96

Fine pine bark mulch $0.75$0.82 $24$27 $1.25$2.10 $40$68

Compost or humus $1.40$1.67 $45$54 $1.50$2.25 $48$72

Mushroom compost $1.50$1.75 $48$56 $2.50$5.35 $80$172

Composted manure $2.95$5.00 $95$160

Miracle-Gro garden soil $3.50$4.77 $112$153

Peat moss $3.80$4.73 $122$152

Sta-Green vegetable soil $4.00 $128

Miracle-Gro Organic $4.00$4.77 $128$153

* Prices based on spring 2009 retail data


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by turning the soil with a shovel when moistureis sufcient to soten the soil. By leaving the soilsurace rough, roots are more likely to grow intothe natural soil once they grow through the soilmix; as a consequence, the soil mix will more easilydrain. Adding about 20 pounds agricultural gypsumper 100 square eet (~4 cups per 4 4 area) to thenatural soil will help with deep root growth and willsupply extra calcium to the soil. (Check soil pH andadd lime as needed.)

Several types o amended soil mixes can be used,but any o them should include good topsoil andlots o organic matter (ground pine bark, peat moss,compost, rotted leaves, etc.). This gives a plantingmix that drains well and is easy to till. I you wish touse a potting soil mix, several options are available.

These may be mixed and added to the raised bed ormixed in the raised bed by hand or by using a smallgarden tiller. See above.

Beore adding the soil mix, loosen or spade theexisting soil and leave it rough. This is easily done


Potting Mixes or Raised Beds and Container GardensThefollowingpottingmixesarewidelyusedinthecommercialhorticulturalindustryandshouldbesuitableforhome

gardentrials.

Potting mix for container-grown, woody ornamentals and planting beds:

6partspinebarkmulch

1 part sand

10lb.grounddolomiticlimestonepercu.yd.(cuppercu.ft.)

14lb.slow-releasefertilizerhighinN(e.g.,18-6-12)withmicronutrientspercu.yd.(~cuppercu.ft.)

AlwayschooseafertilizerwithaboutthreetimesmoreNthanP2O

5(e.g.,3:1:xratioofN:P

2O

5:K

2O

Sta-Green18-6-12orOsmocote19-6-12).

Useabout10%compostinmix.

Potting mix for tender annuals and container-gardening:

4 parts pine bark

1 part peat


14lb.slow-releasefertilizerhighinN(e.g.,18-6-12)withmicronutrientspercu.yd.(~cuppercu.ft.)

AlwayschooseafertilizerwithaboutthreetimesmoreNthanP2O

5infertilizer.

Useabout10%compostinmix.

Potting mix with slow-release fertilizer for seedlings and tender annuals in greenhouse:

1 part peat

1parthorticulturalvermiculite


3lb.premiumgrade13-13-13withmicronutrientspercu.yd.(cuppercu.ft.)

5lb.Osmocote(14-14-14)percu.yd.(1/3cuppercu.ft.)

Some useful measurements:

15-gal.plasticbucket1cu.ft.

5-gal.plasticbucket0.8cu.ft.


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Spade or till 6 to 8 inches deep. Next, blend about2 inches o the soil mix into the upper ew incheso native soil. This will help avoid problems thatcan arise rom having two very dierent soil layersabruptly meeting. Then, fll the rest o the raised bed.

Adjust the pH o the soil mix as needed. The resultwill be a oot or more o rich soil or plants to grow

in. (You may need to replace 1 or 2 inches o the soilmix each year as the organic components begin tocompost and shrink.)

I you plan to use the naturally occurring topsoilwith a minimum o soil amendments, then a soiltest is necessary or applying ground limestone andertilizers. I one o the soil mixes listed on page 5is used, ground limestone and ertilizers are alreadyadded to the mix.

Plant Spacing

For a truly productive raised bed garden, thegardener must relearn many aspects o planting.Gone are the long straight rows and wide spacingbetween rows. Raised bed gardens use space moreefciently to maximize production. In act, rowsmay not even be used. Block planting, with properspacing between plants, is used to optimize yield(see fgure 1). For example, large crops such astomatoes or squash will need to be on 24-inchcenters, whereas cabbage, potatoes, or corn are

spaced 1 oot apart. Medium-sized vegetablessuch as snap beans, peas, or onions are plantedabout 4 to 6 inches apart. Small crops such as theleay greens and root crops can be planted justby scattering the seeds over a small section o thesoil. Staking or caging the plants will allow orthe highest plant density. Other devices can be

used to increase plant density. Trellises and otherstructures can be made to let vine crops and otherplants grow up instead o sprawling.

Efcient Use o Space

An efcient gardener will use the bed to itsullest potential. It is important to plan careully toachieve maximum beneft. Try to group vegetablestogether based on their maturity time or their lengtho productivity (see fgure 1). Plant all short-seasoncrops in one area so that when they fnish producingthey can be replaced by another crop. This isreerred to as succession planting. For instance,plant lettuce, spinach, radishes, and other leay cropsin one area so that the area can be replanted withbeans, cucumbers, or some other warm-season cropater the frst crop is harvested. Also, interplantingcompatible crops to use empty bed space is agreat practice. For example, you may want to plantpeppers or tomatoes between rows o onions. By thetime the onions are harvested, the other plants will

just be reaching a large size. Succession planting and


SquashTomatoesBeansCorn

Figure 1.Exampleofblockingplantingvegetablespacingandgrouping

4

feet

8feet


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interplanting will help you reach the ull potential othe bed. Do not overlook all and winter gardening.Most crops that produce well in the spring months

will do much better in the all (e.g., broccoli, cauli-ower, cabbage, carrots, rutabaga, collards, turnips).

Planting Dates

You might be able to plant a raised bed garden alittle earlier than a conventional garden becauseraised beds warm up more quickly in the spring.By using mulches and row covers, you could plantas much as 2 weeks earlier than a traditionalgarden. Row covers can also extend your harvestlater into the all and early winter.

WateringThe soil mixes used in raised beds tend to drainexcessively the frst year ater establishment. This

will require more requent watering than traditionalplantings. Check the soil and water whenever it is dry,about 2 to 4 inches deep. This could require up to 2inches o irrigation water per week i it doesnt rain.However, you might be irrigating with a soaker hoseor drip irrigation rather than with a sprinkler. In thiscase, you should water until the entire bed is moist.

Drip irrigation oers many benefts, including less

water and more efcient water use. Drip tubing orsoaker hoses can be purchased at local nurseriesand garden centers. The tubes or hoses are then laidout over the bed, spaced about 2 to 3 eet apart,depending on how porous the soil is. Using verylow pressure (7 to 10 psi), the water slowly dripsor oozes rom the hose and flters down into thesoil. Soaker hoses will work at typical household

water pressure. Both options place the water at theroot system, which allows or less evaporation and

prevents water rom moving to nontarget areas, suchas the path. Drip tubing can also be buried below thesoil surace or the most efcient delivery methodand to help avoid animal damage.

Ater the gardens frst year, you will notice that thesoil mix begins to hold more water as the organic

matter gradually composts and turns into humus.You could need less irrigation as the native soilimproves and plants grow deeper roots.

Fertilization

Fertilization needs o a raised bed garden are similarto those o a traditional garden. For mineral soils andsoils based largely on the natural topsoil, start witha soil test. For a soil testing medium in phosphorus(P) and medium in potassium (K), a general recom-mendation would be to use about cup o 13-13-13ertilizer per 4 4oot bed at planting, and anadditional 1/3 cup o 13-13-13 a ew weeks later orlong-season crops. For green, leay vegetables, usea ertilizer high in nitrogen (the frst number on theertilizer bag) as needed to maintain vigor and greencolor. Overertilization is always a concern whenraising vegetables.

I one o the soil mixes listed on page 5 is used, thenthe mix contains adequate ertilizer or the frst ewmonths o the bed. Aterward, use a ertilizer highin nitrogen (N), low in phosphorus (P), and high inpotassium (K), such as a 15-0-15 at a rate o1/3 cupper 4 4oot bed.

Organic ertilizers such as composts, rotted manures,bone meal, blood meal, fsh meal, and cottonseedmeal can be used instead o the inorganic ertilizerslisted on page 5.



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Shane Harris,Regional Extension Agent, Ho Go, Ga, a Ho P, a CharlesMitchell, Extension Specialist, Poo,Agooy a so, oh wh A uy. AapoRaised Bed GardeningyDennis Patton aWard Upham (Kaa sa uy, 2006). th wa aap o Cha mch, rayo K, a Cha Ga Potting Mixes forContainer Gardens(ACes, 2006).

Appcao xp o h a, Bruce Dupree.

For more information, ca yo coy exo ofc. look yo pho coy yo coy a o f h .

ta a a a h pcao a g o oao ppo oy. nogaa, o, o cao aog copaa poc o p yh Aaaa Coopa exo sy.

Ph y h Aaaa Coopa exo sy (Aaaa A&m uy a Auy), a qa oppoy cao a poy.

15m, Reprinted April 2012, Anr-1345 2012 y h Aaaa Coopa exo sy. A gh .

ANR-1345

Mulches

Summer mulches such as straw or mini pine barknuggets should be placed around vegetable plantsto help conserve moisture, cool the soil, and controlpesky weeds. Apply a 2- to 4-inch layer around

young and emerged plants, and over the soil aterit has warmed; do not apply too early as you might

keep the soil cool and slow the growth o warm-season crops. Old newspapers can also be recycledand used as ree and biodegradable mulch in the

vegetable garden. Layers o newspapers can be

placed down prior to adding other mulch. In a year,the newspaper will break down, adding valuablenutrients back into the soil.

Raised bed gardening allows or more efcientuse o space to maximize your investment o time,energy, and money. As with any gardening product,

the un and rewards come rom your own experi-mentation and rom fnding techniques that workbest or you. For more inormation, contact yourlocal county Extension ofce, or visit us online at

www.aces.edu.

Raised Bed Gardening 1

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