Plastic Mulches

V. A. Clarkson W. A. Frazier

- Statin Bulletin 562 Septemer 1957 CorvaMk Agricultural Experiment Station-'Oregon State College Plastic Mulches

By V. A. Clarkson and W. A. Frazier*

WHAT IS PLASTIC OR POLYETHYLENE? Polyethylene is a thermoplastic resin which is pliable, chemically inert, odorless, and nontoxic. Moisture does not penetrate it, but certain gases such as car- bon dioxide and oxygen easily pass through. The plastic is available in several colors, including clear, white, and black.

WHAT WILL IT DO? Weed control, soil moisture conservation, crop yields, and fruit-disease control of warm-season horticultural crops have all been increased or improved. This is based upon research trials with black plastic mulch at the Oregon Agricultural Experiment Station. Use of the plastic also may increase soil temperature, improve fruit cleanliness, and decrease soil compaction.

WHAT ARE ITS LIMITATIONS? Polyethylene plasticis moderately expensive and may not pay on all crops. This, and other items such as cost of laying, handling, and hand planting, mean that it may be used to best advantage on high-value crops such as vegetables, small fruits, and orna- mentals.

* Research Assistant in Horticulture and Horticulturist, Oregon Agricultural Experiment Station.

2 Which Mulch To Use? A mulch is a material spreadon the Mulching materials do not have the ground surface to protect a plantorsame characteristics. Consider the var- plant roots. Most often a mulchpro- ious properties before choosing a suit- tects a plant from low temperature in-able mulch. The use of one material jury and moisture loss by wind andin preference to the others depends soil evaporation. It may be applied toupon crop requirements, costs, and the control weeds, aid soil structure andresults desired. drainage, and improve fertility. If the incorporation of organic ma- Use ofnatural organic materialsterial into the soil is a primary objec- such as straw and manure has beenan tive,use manure, straw, or another acceptedhorticulturalpracticeforeasily decomposed organic material. If many years. Straw and other similarvalue as a fertilizer also is important, materials are widely used as a winterstraw may not be an acceptable ma- protection for low-growing plants, andterial. If growing warm season crops, manure is used primarily as an organicsoil temperature and moisture are im- fertilizerwithincidentalmulchingportant,blackpolyethylenefilm properties if left on the soil surface.which absorbs heat and is impervious in the Pacific Northwest, sawdustto moisturemay be especially useful. often easily obtainedis being usedas The table on page 11 shows some a mulch material on numerous small-of the characteristics of various mulch- and tree-fruit crops as wellas orna-ing materials, such as sawdust, straw, mentals. manure, paper, and black plastic.

Use Proper Width and Weight To insure adequate weed control andberries, 4-mil (.004") plastic may last moisture conservation, as wellas tolonger than 2-mil plastic. protect ripening fruit from soil con- Most papers used for mulching are tact, use plastic that is 3 or 4 feet wide.sold in rolls 3 feet wide. Under the in some instances itmay be desirablemoist weather conditions of the Wit- to use a plastic that is considerablylamette Valley, all of the paper mulches wider to protect fruits suchas melonstesteddisintegrated within a year's and squash from rot. If the plastic istime. Plastic, which may be purchased removed each year, 1(.0015") or 2in various widths, have lasted 3 or more (.002") mils in thickness is adequate.years in experiments at the Oregon With perennial crops suchas straw-Agricultural Experiment Station.

3 How to Apply Plastic Mulch.

LEVEL toil before apply- 1 I ing plastic. Seedbed must be well workedtilled and rolled.Before laying polyethylene mulch, be sure to irrigate soilifitis dry to insure adequate moisture for seed germination and seed. lingortransplant growth.

PLACE a band of ferti- 2 lizerbeforeyoulay plastic. Withannual vegetable crops, be sure to apply enough tolast throughout thegrowing season.Use stringlinetomarkrow center. Place fertilizer 3 or 4 inches under soil and to sideof seedsorplants.

APPLYplastic,using 3guide line as aid for maintaining straight row. Place stake at end of row to mark fertilizer band. Workisunderway tode. velop atractor attachment formechanical layingof plastic for commercial use. COVERplasticedges 4withsoiltoprevent blowing. Plastic may need weights, such as bricks, laths, or large wire staples drivenintosoiltohold mulch down in windy areas. Growingplantswillhelp hold polyethylene inplace.

CUT hole in plastic 5large enough for transplanting. Water plants well. Ifseedinginhills,use a corn jabber or similar planter. When rowplant- ing, remove a plasticstrip

1 inch wide by 2 to 3 feet longfromcenter,leaving a 3 to 6 inch crosspiece of plastic between the strips.

GROWING plantsin 6plastic is similarto normalc u It u r a Ipractices. Check plants far water needs, insect and disease damage. Transplanted head lettuce at right has been growing for 6 weeks.Irrigationwater has carried soil onto plastic. Use Normal Plant Spacing When planting through a polyethyl-a matted row, and the runners allowed ene mulch, normal planting distancesto root in between the mother plants. are usually used. One or more rowsThe use of polyethylene makes it im- of plants may be placed in a singlepossible for these runners to root into sheet of plastic. Strawberries in thethe soil and a hill system of planting Pacific Northwest are often planted inmust be used.

Plant Spacing in Plastic Mulches Distance Between Method of Plants Between Between plantsin planting per hill hills rows row Feet Feet Feet Bush beans hill 3-4 3-4 Cabbage row 4-5 2-3 Cantaloupe hill 2-3 2-3 4-5 Cucumber hill 2-3 2-3 4-5 Eggplant row 3-4 l1

Pole beans hill 3-4 l-} S Strawberries5 row 3 Summer squash.. hill 2-3 2-4 5 Sweet pepper row 4-5 l-2 Tomato row 4-5 2-3 Watermelon hill 2-3 4-6 6 Winter squash hill 2-3 4-6 6-8 * A double row of plants 1 foot apart on a 4-foot wide sheet of plastic.

Plastic Controls Weeds There is no advantage in cultivatinghand weedings. After weeds have been under the plastic sheets once they areremoved, competition by the crop is applied. Soil does not compact readilyusually enoughtodiscourage most beneath the mulch, and if the blackweed growth. polyethylene is used there is no serious 1vVeed control is necessary between weed growth. There will be some weedstrips of plastic. Use a hoe or other growth in the immediate area of theshallowcultivatingequipment.Be holes or slits made in the plastic. Thiscareful not to cut the edges of the plas- usually means one or possibly twotic while cultivating.

6 With some crops, a weed controlthe Oregon State College experimental chemical between the strips of plasticfarm, white polyethylene thin enough may help. Since plastic is chemicallyto allow low amounts of light penetra- inert,theapplicationofchemicals tion did not control weeds. With low should not affect it. If a weed killer islight transmission and warm growing applied after plants are up, be sureconditions underthewhiteplastic, that no spray or spray drift comes inweeds grew remarkably welleven- contact with them. tually tearing the plasticmaking it In the trials of plastic materials atuseless as a mulching material.

Mulched tomatoes, left, show weed control possible with plastic. lJnmulched tomatoes, right. One hand weeding of mulched plants is often necessary to control weeds that grow through transplanting slit.

Less Irrigation With Plastic? Much remainstobeunderstoodage into the slits or holes in the plas- about irrigationof polyethylene- tic where plants are located. mulched plants. Experience has shown Intervals between irrigations may be that with normal sprinkler irrigation,spread out considerably with polythy- water does not limit production. Howlene, but the demands of the various easily irrigation water enters the soilcrops and climatic conditions will de- under the plastic depends upon drain-termine the frequency of irrigation.

7 Control Diseases and Insects Diseaseandinsectcontrolontions common in some low-growing mulched plants should be about thehorticultural crops is the loss of fruit same as under nonmuiched conditions.to various soilborne organisms. When The plants are equally accessible forpaper or polyethyleneisused as a dusting or spraying of foliage preda-mulching material, direct contact be- tors. The effect mulches may have ontween the fruit and soil is broken. This soilborne diseases, and insects are nothas resulted in a lower percentage of yet known. rotted fruit in crops such as tomatoes, One of the important disease condi- melons, and strawberries.

Plastic Mulches Not Toxic Since polyethylene is a chemicallyin young, low-growing plants such as inert material, it has not been foundmelons, and may result in serious in- to be toxic on the plants tested. Cer- jury or death. This toxicity may be tain of the paper materials used as acaused by the action of heat or water mulch have exhibited marked toxic ef-on the tar or creosote impregnants used fects on some plants. Symptoms ofin some papers. Conservatively used, toxicity are usually more pronouncedorganic mulches are non-toxic.

Use Normal Harvesting Methods The harvest of plants grown on aing or driving of rubber-tired machines polyethylene mulch is the same as withover the plastic does not usually tearit. nonmulched crops. The material resistsThis is not true with some types of tearing, although it may be easily cutpaper. Paper becomes brittle on expos- with a sharp instrument. Normal walk-ure, while plastic remains pliable.

Plastic Pays With Higher Yields Types of plants which will respondmulched plants and the amount of fruit to plastic or paper mulching have notset was much greater than on non- beencompletelyinvestigated.Most mulched treatments. The possibility of warm season crops such as tomatoes,using black polyethylene on ornamental melons,cucumbers,eggplants,bush crops, both annual and perennial, has and pole beans, and sweet peppers havenot been investigated. There appears to responded well. Cool season crops suchbe no reason why it may not be used as late cabbage showed no increase into advantage on a number of such yield from the use of polyethylene. crops. (See chart, page 9.) Plastic-mulched crops grown on the With warm season crops tested, theOSC experimental farm did not ma- amountoffoliargrowthonthetureearlier than nonmuiched crops.

8 PLASTIC MULCH INCREASES YIELDS U, 0. 0No Mulch Marketable 4-0 C 0Black Poly. Unmarketable (-) In C No Mulch cri wBlack Poly.£ £ £ £ 0

U, 4-0No Mulch 0 EBlack Poly. £ £ *& a***&táj.

0 5 10 15 20 Tons / Acre

Increase in yields of tomatoes mulched with plastic was marked, as shown in the above graph and this photo. Fruit in photo was from one mid-season picking of 6 plants. Fruit rot was better controlled.

9 Yield of cabbage, a cool season crop, was not increased with a plastic mulch. Weed control in mulched plots was excellent. Conditions for cabbage root maggot attacks, however, are better with mulch.

In most cases, however, there wereis applied and the crop planted before more fruits and vegetables harvestednormal planting date,there may be from the polyethylene-mulched plantssome earlinesssincethese mulches in the early harvests because of con-tend to warm the upper soil area, creat- siderable yield increases. If the plasticing better conditions for germination.

Plastic May Be Reused Plastic can be reclaimed and reused.it until next season. But removing vine Initialcost can then be pro rated,or plant growth prior to the removal thereby reducing the cost per year. of the plastic may not pay. It may be In the home garden with a relativelynecessary to burn both the plastic and small area covered by plastic, the ma-plant growth with a weed burner or terial may be easily cleaned off, rolledother similar tool. It is not known now up, and stored until the next season.whether itwill be more practical to The home or commercial gardener alsopick up the plastic or leave it in place may be able to leave the plastic in placefor several years. Plowing under plas- andreplantthefollowingseasontic is difficult and it tends to foul culti- through the same holes. vating equipment. Since it does not dis- The commercial vegetable growerintegrate easily,it will remain in the also can pick up the plastic and storesoil for a long time.

10 Characteristics of Mulching Materials

Black poly- Characteristics Sawdust Straw Manure Black paper ethylene Longevity Decomposes Not more than Not more than One growing Two or about"-" one year one year season or less per year more years Water retention (reduction of surface evaporation) Good Poor Poor-fair Good Good (impervious to water) Eleat absorption Poor Poor Poor Good Good 3oil heat retention Fair Poor Fair Good Good Weed control Fair Poor Poor Good Excellent Water penetration Good Good Good Fair-poor None ?ertilizer value Poor Poor Good None None extra nutrients needed for decomposition Nitrogen Nitrogen None None None Coxic qualities Usually none None Possibility, if If oil, tar, or None applied too creosote im- heavily pregnated ncidence of rotted fruit over nonmulched plants Lower Unknown Unknown Lower Lower (probably lower) Cleanliness of fruit Good Good Poor Excellent Excellent )urability (can it be walked on?) Good Poor Poor Poor Good Fffect on soil structure Poor Good Good Unknown Excellent Lppearance of mulch Good Poor Poor Good Good Soeeëd'2 A Black Plastic Mulch?

l/e4if you want to . grow warm season row crops.

grow high value crops.

help control weeds.

conserve soil moisture.

help control soil-contact fruit rot.

maintain fruit cleanliness.

slow soil compaction.

cost of material is too great.

cost of applying and hand labor is too great.