/. 7 7/ .23/ Salt Injury to Ornamental Shrubs and Ground Covers ,^\ UNITED STATES («Hw) DEPARTMENT OF w - f> AGRICULTURE HOME AN! GARDEN NUMBER 231 PREPAREDLY J, SCIENCE' AND EDUCATION : ADMINISTRATION Contents 3 How soils become saline 4 How salinity affects plants 6 Confirming salt injury 6 Plant selection is important 8 Reducing soil salinity 10 Important points to remember Department publications contain public information. They are not copyrighted and can be reproduced in whole or in part with or without credit. This bulletin supersedes Home and Garden Bulletin No. 95, Reducing Salt Injury to Ornamental Shrubs in the West, May 1964. Issued July 1980 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Salt Injury to Ornamental Shrubs and Ground Covers Leland E. Francois, SEA research agronomist 1 If the ornamental shrubs around your home exhibit leaf burn or fail to grow normally despite an adequate fertilizer and irrigation program, the trouble may be due to salinity. Salts of various kinds are found in more soils and many are essential to plant growth. However, some soils contain an overabundance of salts, which at high concentrations can damage plants. Salts containing sodium or chloride are particularly injurious to ornamental shrubs. How Soils Become Saline Salts are usually carried to the soil in water. Water from rivers and wells always contains some dissolved salts. When water containing salts is used for irrigating plants, the water is taken up by the plants or evaporates from the soil surface, but most of the salt is left in the 1|J.S. Salinity Laboratory, Riverside, CA 92502 soil. Consequently, repeated light watering without drainage can result in considerable salt accumulation in the soil around the roots. The saltier the water, the faster salts accumu- late in the soil. Potted ornamentals may also be afflicted by salt accumulation. If only enough water is added to replace that lost by plant use or evaporation from the potting soil, injurious concentrations of salt will eventually occur. Fertilizer, applied in amounts greater than necessary to meet the plant needs, can be as harmful as sodium or chloride in the soil. Ade- quate fertilization is necessary for healthy shrub growth, but too much causes more harm than good. A high water table can also con- tribute to excess salt in the root zone. Evaporation of water from the soil causes ground water to move upward toward the surface, carrying salts into the upper soil level. In most of the United States, rain- fall normally flushes excess salt out of the root zone. In areas where the rainfall is less than 20 inches, as in many areas of the Western United States, the excess salt is likely to accumulate in damaging amounts unless preventive measures are taken. How Salinity Affects Plants Salinity generally causes stunting of plants. All plant parts— leaves, stems, roots, and fruits— are smaller than normal. The higher the salinity, the more the plant is stunted. Sodium and chloride, the two ele- ments most common in saline soils, may cause specific injury to certain plants. The accumulation of these elements in many ornamentals may cause leaf burn, premature leaf drop, or stem dieback. Salt-sensitive plants may be killed. The color illus- trations show symptoms typical of sodium and chloride injury. Salinity can injure ornamentals in- directly. The weaker shrubs may be less able to resist frost injury, disease, or insect infestation. Salt injury is generally more severe during periods of hot, dry weather. High temperatures intensify leaf-burn injury caused by sodium or chloride. Salt also is likely to ac- cumulate in the soil at an increased rate during hot weather. Water loss from the soil— through plant use and evaporation— is greatest then, and ordinary watering may not be heavy enough to leach salts from the root zone. Sprinkling with saline water dur- ing the day time when evaporation is high may be potentially more in- jurious to ornamentals than surface irrigation. Leaves wetted by the sprinkling water absorb salts directly through their surface and injury may exceed that expected from soil salinity. Frequent, light sprinklings with saline irrigation water should be avoided to prevent any buildup of salt on the leaf surface. When foli- age is sprayed, sufficient water should be used to wash excess salts from the leaves. Other sources of salt spray encountered in some areas include salt drift from ocean surf and deicing salts splattered on plants along streets by automobiles. Oriental arborvitae .V * f~w+>. n OL3. H +**SL-/-?- ii9 ^N# Chine Leaf chlor 3# m Ji se holly injury typical of so ide accumulation. dium and r f3£T Chinese hibiscus Rose Laurustinus ow Salinity Is Measured Soil salinity is usually deter mined by measuring the elec- trical conductivity of the soil solution. A soil sample is satu- rated with distilled water, which mixes with the salt in the sample. The salt solution is then extracted from the sample and tested for its capacity to conduct an electric current. The saltier the soil solution, the greater its con- ductivity. Conductivity, and hence soil salinity, has commonly been expressed in millimhos per centimeter (abbreviated as mmhos/cm). The metric unit for electrical conductivity is deci- siemens per meter (abbreviated as dS/m). One dS/m equals one mmho/cm. How much salt is repre- sented by a conductivity of 1 dS/m? It represents about 2 level teaspoons of table salt dissolved in 5 gallons of wa Pineapple guava Confirming Salt Injury If you suspect salt has injured your shrubs, it is well to confirm the cause of damage before beginning to correct it. Leaf burn and stunting are likely symptoms of a salinity problem but they may also be caused by drought. To determine whether the damage is caused by salinity, you may want to have your soil and irrigation water tested. Leaf samples may also need to be tested at times to confirm a salinity problem. Your county agri- cultural agent or State Agricultural Experiment Station can tell you where to have these tests done. Be sure that the soil sample taken is representative of the soil mass in which the roots are growing. Samples from ridges or surface soil where salts tend to accumulate may not give a true representation of the root area. Plant Selection Is Important Proper shrub selection can mean the difference between success and failure in landscaping when irri- gating with saline water. The more saline the irrigation water is, the fewer the number of plants available for selection. The following table presents the salt tolerance of 41 shrubs, trees, and ground covers tested at the U.S. Salinity Labora- tory, Riverside, Calif. Common name Botanical name High Tolerance (Soil salinity no higher than 10 dS/m) Croceum Iceplant Purple Iceplant Rosea Iceplant White Iceplant Ceniza Brush cherry Bougainvillea Natal plum Hymenocyclus croceus Lampranthus productus Drosanthemum hispidum Delosperma alba Leucophyllum frutescens Syzygium paniculatum Bougainvillea spectabilis Carissa grandiflora Good Tolerance (Soil salinity no higher than 8 dS/m) Aleppo pine European fan palm Rosemary Spindle tree Blue dracaena Oleander Weeping bottlebrush Pinus halepensis Chamaerops humilis Rosmarinus lockwoodii Euonymus japonica Cordyline indivisa N eh urn oleander Callistemon viminalis Common name Botanical name Moderate Tolerance (Soil salinity no higher than Spreading juniper Pyracantha Thorny Elaeagnus Oriental arborvitae Indian hawthorn Japanese black pine Dodonaea Xylosma Japanese boxwood Yellow sage Glossy privet 5 to 6 dS/m) Juniperus chinensis Pyracantha fortuneana Elaeagnus pungens Platycladus oriental is Raphiolepis indica Pinus thunbergiana Dodonaea viscosa Xylosma congestum Buxus microphylla Lantana camara Ligustrum lucidum Poor Tolerance (Soil salinity no higher than 3 to 4 dS/m) Compact strawberry tree Arbutus unedo Laurustinus Viburnum tinus Chinese hibiscus Hibiscus rosa-sinensis Heavenly bamboo Nandina domestica Japanese pittosporum Pittosporum tobira Algerian ivy Hedera canadensis Very Poor Tolerance (Soil salinity no higher than 2 dS/m) Southern yew Glossy abelia Photinia Rose Chinese holly Pineapple guava Oregon grape Star jasmine Pyrenees cotoneaster Podocarpus macrophyllus Abelia grandiflora Photinia fraseri Rosa Ilex cornuta Feijoa sellowiana Mahonia aquifolium Trachelospermum jasminoides Cotoneaster congestus If the soil salinity exceeds the limits given in the table, injury such as severe leaf drop, leaf burn, and/or stunting will usually occur. Shrub size makes little difference in ward- ing off the injury. Frost, heat, smog, and drought tolerances should also be con- sidered in selecting shrubs suitable for your area. All of these environ- mental factors can weaken the plant and thus make it less able to with- stand the detrimental effects of salinity. Reducing Soil Salinity To reduce soil salinity to a level that your shrubs can tolerate, irri- gate heavily. This will leach the ac- cumulated salt down into the soil below the root zone. How heavily you irrigate will de- pend upon how saline the soil is and the depth of soil to be leached. If you excavate a shallow basin around the plant, and water the plant within that basin, then you will be able to judge the depth of the water entering the root zone. In general, for each foot of soil to be leached— • 6 inches of irrigation water will leach out about one-half of the salt. • 12 inches of irrigation water will leach out about four-fifths of the salt. • 24 inches of irrigation water will leach out about nine-tenths of the salt. Remember though that the salinity of the water in the soil can never be less than the salinity of the irriga- tion water. After the soil salinity has been reduced to a tolerable level for your shrubs, continue to apply extra water periodically when irrigating to prevent a new buildup of salt. The saltier the irrigation water and the more salt sensitive the shrubs, the greater is the amount of water needed for irrigation. The following table is a guide to the depth of water required for maintaining a safe level of soil salinity when 3 inches of water are lost by evapora- tion and plant use. During hot sum- mer months, this loss of water could be expected every 10-12 days. Care must be taken if heavy irriga- tions are required, because too much water can be as harmful to the plant as the soil salinity. This is particularly a problem with poorly drained soils. Poor drainage may sometimes be caused by excess sodium which can be removed by adding gypsum to the soil. The soil-test report should tell you if the soil contains too much sodium and should give direc- tions for correcting the problem. If drainage is poor and excess sodium is not the cause, installation of drain tile may be necessary to remove the salt-laden water from beneath the root zone. Example When growing Aleppo pine, soil salinity should not exceed 8.0 dS/m, according to the plant selection table. The irrigation guide shows that when irrigating with 1.0 dS/m water, a total of 3.4 inches of water must be applied to keep soil salinity from exceeding 8.0 dS/m. If drip ir- rigation is used, the same total amount of water should be applied over the 10- to 12-day period as would be applied with a single flood or sprinkler irrigation. Irrigating Ornamental Shrubs: Guide to depth of water required for maintaining safe level of soil salinity Maximum soil salinity tolerated by shrubs (Conductivity of saturated extract in dS/m). i> O Salinity of irriga- tion water (Con- ductivity in dS/m) I (rain water) 0.5 1.0 2.0 Required depth of irrigation water to provide adequate leaching and to replace 3 inches of water lost by evapo ration and plant use. 3.0 3.3 3.8 3.0 3.0 3.0 3.0 3.2 3.4 0.3 3.6 :: 40 4.0 6.0 4.0 4.5 6.0 ■l Important Points to Remember • Confirm that the damage is from salinity and not from some other cause. • Select the right shrubs for your conditions. • Leach with enough water periodically to prevent salt accumula- tion. • Plants can withstand salinity better when healthy— so fertilize adequately and control insect pests. 10 UNIVERSITY OF FLORIDA 1262 08850 4062 U.S. DEPARTMENT OF AGRICULTURE SCIENCE AND EDUCATION ADMINISTRATION WASHINGTON, DC. 20290 OFFICIAL BUSINESS PENALTY FOR PRIVATE USE $300 POSTAGE AND PEES PAID U.S. DEPARTMENT OP AGRICULTURE AGR 101