Even though the application of gravel mulches is straight forward, many of us do not fully understand their consequences.

Gravel mulches have the lure of low maintenance. Many a homeowner has installed it in the hopes that once installed, everything is perfect forever. I dont believe anything can be further from the truth.

Whenever installing gravel mulches, its very important to use a high quality spun bonded or woven polypropylene fabric to separate the gravel from the underlying soil. While fabrics may inhibit the proper growth pattern of many plants, it is by far easier to deal with gravels under laid with a fabric than with nothing. Avoid using plastics as they dont allow for the soil to breath, fertilizer to get to the plants roots and water to percolate properly, except through the seams, edges along the foundation and through tears and holes.

Some of the major drawbacks to gravel mulches include the following:

1. All to often people try to match the brick or color of their homes to the gravel mulch. While in theory this sounds good, in reality it makes it appear that their beautiful brick or walls are crumbling into the landscape.

2. Or, we try a contrast to the beautiful colored brick or walls by installing very light or pure white gravels. All you notice when you drive by or up to the home is this white border of rock. It stands out like hot coals. Your front entrance and flowers should be what draws attention to your home. Not the color of the mulch.

3. For many plants, different colors of gravel have very different effect on growth. Most of the plants we utilize in our landscapes did not evolved in gravel beds and have a difficult time dealing with it. This is not Arizona.

4. The gravel is going to be there a long time, so chose carefully.

Trying to keep gravel clean of debris in the long term is almost impossible. The cavities between the gravel collect just about everything the wind blows and everything that falls from the plants in the area. While yard blowers remove the larger pieces from the uppermost level, the smaller pieces remain lodged in the cavities. These organic particles and mineral soil debris build up fairly quickly and produce a nice fertile environment for weeds. Just how do you clean a gravel bed? You pick up ALL of the gravel, screen it, wash it and relay it. Glad you have the time to do this.

The shape of the gravel is another thing.

Round or smooth gravels do not pack well and continuously flow around the landscape. The larger the gravel size, the more it moves. Crushed gravels pack somewhat better. The angular shapes lock into place better, especially if there is a mixture of sizes.

The composition of gravel is a whole other matter.

The best gravels should be relatively durable and impervious to weather. Granites and other related dense hard gravels retain their shape and color. Besides melting or crumbling, more porous stones such as sandstones and limestones can harbor different fungi, mosses and other discoloring agents.

Limestone gravels such as White Marble, Washed White, Lime Screenings, Roadway gravels and most other light colored gravels in this part of the country are composed primarily of Calcium Carbonate aka Limestone. While this is the cheapest stone you can purchase in this part of the state, it also dissolves quite rapidly when exposed to water. Since our rainfall is slightly acidic, the limestone reacts with our rain and meltsthe stone. If you dont realize how quickly this actually happens, take a look at the new rock outcrops along a new highway and then look again a few years later. As the stone dissolves, cavities result, some expanding to become very large, such as the Cave of the Mounds. Didnt you ever wonder where the limestone gravel or sand went that allowed a cavity to form under your driveway? This disintegration of the limestone gravels under your walks, driveway, patio, and around your trees, shrubs and flowers changes the chemistry of the soil into an alkaline or higher pH medium.

Many commonly used landscape plants are calcicoles, preferring soils with a pH over 7.0. Since each full numerical increase in the pH number represent a ten fold increase, a rise in pH from 7.0 to 8.0 represent a ten fold increase in the alkalinity. Nutrient availabilities vary considerably with varying pH. The availably of Iron (Fe), Manganese (Mn) and Magnesium (Mg) are hampered in high pH soils; hence chlorosis is common on plants that dont like high pH or alkaline soils. The following list is not an endorsement of these plants for use in the landscapes. Some are bona fide weeds. Since only tolerant or lime loving trees, shrubs and bushes are listed, you may surmise that almost all of the other non-listed woody plants are intolerant of pH over 7.0 and prefer acidic soils or those with a pH below 7.0.

Some plants preferring or adaptable to limey soils (pH over 7.0) include the following. Those with ** will show chlorosis if the pH or alkalinity of the soil gets over a pH of 8.0 or so.

Acer ginnala Amur Maple **
A. negundo Boxelder
A. platanoides Norway Maple
A. saccharinum Silver Maple **
A. saccharum Sugar Maple **
Aronia arbutifolia Red Chokeberry
A. melanocarpa Black Chokeberry
Berberis thunbergerii Barberry
Betula Whitespire Birch **
Buxus Glencoe Boxwood
Celtis occidentalis Hackberry
Cercis canadensis Redbud
Cornus alba Redtwig Dogwood
C. mas Corneliancherry D.
C. racemosa Gray D.
C. sanguinea Bloodtwig D. **
C. sericea Redosier D.
Corylus americana American Filbert
C. colurna Turkish F.
Cotinus coggygria Smokebush
Cotoneaster apiculatus Cranberry Coto- neaster
C. divaricatus Spreading C.
C. horizontalis Rock C.
C. lucidus Hedge C.
Deutzia gracilis Slender Deutzia
Diervilla spp Diervilla
Euonymus alatus Burning Bush
E. europaeus Spindle Tree
E. bungeanus Winterberry E.
E. fortunei Wintercreeper E.
Forsythia Forsythia
Fraxinus pennsylvanica Green Ash
Ginkgo biloba Ginkgo
Gleditsia Honeylocust
Gymnocladus dioicus Kentucky Coffee tree
Hamamelis vernalis Vernal Witchhazel **
H. virginiana Witchhazel **
H. x intermedia Witchhazel **
Hibiscus syriacus Rose of Sharon
Hydrangea arborescens Smooth Hy drangea
H. a. petiolaris Climbing H. **
H. paniculata Panicle H.
H. quircifolia Oakleaf H. **
Hypericum prolificum Shrubby St. Johnswort
Juniperus chinensis Chinese Juniper
J. communis Juniper
J. horizontalis Creeping J. **
J. procumbens Japanese J.
J. sabina Savin J.
J scopulorum Colorado J.
J. squamata Singleseed J.
J. virginiana Eastern Redcedar
Kerria japonica Japanese Kerria *
Kolkwitzia amabilis Beautybush
Ligustrum amurense Amur Privet
L. vulgare European P.
Maackia amurensis Amur Maackia
Malus Crabapple
Morus Mulberry
Paxistima canbyi Paxistima
Phellodendron amurense Amur Corktree
Philadelphus coronaries Mockorange
Physocarpos opulifolius Ninebark **
Picea abies Norway Spruce **

P. glauca White Spruce **
P. omorika Serbian Spruce
P. orientalis Oriental Spruce
P. pungens Colorado Spruce
Pinus bungeana Lacebark Pine **
P flexilis Limber Pine **
P. mugo Mugo Pine
P. strobus White Pine **
Platanus x acerfolia London Planetree
Potentilla fruticosa Bush Cinquefoil
Prunus besseyi Sand Cherry
Quercus macrocarpa Bur Oak
Q. coccinea Scarlet Oak **
Q. ellipsoidalis Hills Oak **
Q. robur English Oak
Rhodotypos scandens Black Jetbead
Rhus typhina Staghorn Sumac
R. glabra Smooth Sumac
Ribes alpinum Alpine Currant
Robinia pseudoacacia Black Locust
R. hispida Roseacacia Locust
Rosa rugosa Rugose Rose **
R. carolina Carolina Rose
R. glauca Redleaf Rose **
R. virginiana Virginia Rose
Sambucus canadensis American Elder
S. racemosa European Red Elder **
Schizophragma hydr. Japanese Hydrangea- vine **
Shepherdia canadensis Russet Buffaloberry
S. argentea Silver Buffaloberry
Sorbaria sorbifolia Rural Falsespirea
Sorus alnifolia Korean Mountainash
Spiraea x bumalda Bumald Spiraea
S. japonica Japanese S.
S. x cinerea Grefsheim S.
S. nipponica Nippon S.
S. prunifolia Bridalwreath S.
S. thunbergii Thunberg S.
S. trilobata Threelobe S.
S. x vanhouttei Vanhoutte S.
Symphoricarpos albus Snowberry
S. x chenaultii Chenault Coralberry
S. orbiculatus Indiancurrant C.
Syringa species & cultivars Lilac **
Taxus baccata English Yew **
T. cuspidata Japanese Yew **
T. x media Alglojap Yew **
Thuja occidentalis Eastern Arborvitae
Tilia americana Linden Basswood
T. cordata Littleleaf Linden
T. tomentosa Silver Linden
Ulmus species & hybrids Elm
Viburnum dentatum Arrowwood Viburnum
V. lantana Wayfaringtree **
V. lentago Nannyberry V.
V. opulus European Cranberry V.
V. prunifolium Blackhaw V.
V. x rhytidophylloides Lantanaphyllum V.
V. sargentii Sargent V.
V. trilobum American Cranberry V.
Weigela florida Weigela
Xanthoceras sorbifolium Yellowhorn
Xanthorhiza simplicissima Yellowroot **

My favorite gravels are Crushed Granites and small Crushed Basalts. They pack relatively well, they dont negatively impact the chemistry of the soil, they re very stable, and if you dont like them at a later date, just till them into the soil as they make an excellent soil amendment loosening hard packed soils.

More mulch info: landscape mulches

originally published newsletter 2003 V17 #2


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