Gardenesque
Plants 
Design Information Clay Soils Harford County, MD
Plants that Spread
While surfing the web for garden information I found the following website that had a great pdf file titled "Spreading Ornamental Plants: Virtues & Vices" from the Purdue University Cooperative Extension Service. This file is no longer available.
It rated the aggressiveness of various spreading plants depending on how fast they spread with a numbering system from 1 to 5. The number 1 was a very slow spreader and 4 and 5 were rated as aggressive spreaders. This type of information is very helpful to landscape designers as well as the homeowner trying to decide if a plant is appropriate for a particular site. The plants marked as slow spreaders with a rating under 3 could be planted anywhere and won't overtake other plants planted with them.
Aggressive plants with a rating of 4 to 5 should be carefully sited to take advantage of their aggressive spreading habits. They should not be planted among slower-growing plants.
The plant's propagation method is listed as well: rhizomes, stoloniferous, and reseeders. A description of each method is given along with information on how to control the plants in each category. "Control measures include regular thinning and division, pruning, confinement and deadheading" as well as removing seedlings in the spring.
At the time I was working on a design for a steep slope with stony, clay soil and this article gave me the answers I needed to cover this slope as inexpensively as possible, and yet, in a few years, have a perfectly wonderful, and interesting planting design.
For the steeper areas:
For the steeper areas that are difficult to get to, I would pick plants that I wouldn't normally include in a design for a typical border on level ground. Some plants that pop out of the list are:
| Botanical Name | Common Name | Light Requirement | Soil Requirement |
|---|---|---|---|
| Aegopodium podagraria | Bishop's Weed | pt sh to shade | any soil including clay |
| Achillea millefolium | Yarrow | sun | dry soil |
| Artemesia ludoviciana | White sage | sun | dry, sandy, well drained soil |
| Chasmanthium latifolium | Northern Sea Oats | sun to pt sh | moist to wet soil |
| Hypericum calycinum | St. Johnswort | sun to pt sh | tolerates dry soil |
| Lysimachia clethroides | Gooseneck Loosestrife | sun to pt sh | moist to wet soil including clay |
| Monarda didyma | Bee-Balm | sun to pt sh | moist, clay soil; tolerates dry soil |
| Oenothera berlandieri | Evening Primrose | sun | infertile, well drained soil |
| Phalaris arundinacea ‘Picta' | Ribbon Grass | sun to shade | any soil including dry |
| Physostegia virginiana | Obedient Plant | sun to pt sh | moist, acid soil |
| Tradescantia virginiana | Virginia Spiderwort | sun to shade | moist soil; tolerates dry soil |
| Rudbeckia fulgida | Black-Eyed Susan | sun to pt sh | clay soil including dry |
Some others from my own database that are not on the Purdue list that I considered were:
| Botanical Name | Common Name | Light Requirement | Soil Requirement |
|---|---|---|---|
| Liriope spicata | Creeping Lily-turf | sun to shade | moist, organic soil; tolerates dry soil |
| Phlox subulata | moss phlox | sun | gritty, well drained alkaline soil |
In some of the easier to get to areas I would pick plants that are not as aggressive and can be planted in small groups.
| Botanical Name | Common Name | Light Requirement | Soil Requirement |
|---|---|---|---|
| Aquilegia canadensis | columbine | sun to pt sh | moist, well drained soil |
| Festuca glauca | fescue | sun | dry, infertile, well drained soil |
| Helenium autumnale | Helen's flower | sun | moist, well drained soil; tolerates dry |
| Heliopsis helianthoides | sunflower | sun to pt sh | well drained soil; tolerates dry |
| Panicum virgatum | Switchgrass | sun to pt sh | moist soil; tolerates dry |
Also, some suckering shrubs might be nice as accents if I give them enough room to grow. they could include:
| Botanical Name | Common Name | Light Requirement | Soil Requirement |
|---|---|---|---|
| Diervilla sessilifolia | southern bush honeysuckle | adaptable to many light conditions | moist infertile soil; tolerates dry |
| Cornus alba 'Elegantissima' | Tatarian Dogwood | sun to pt shade | moist, well drained soil; adaptable to most soils including clay |
| Eleutherococcus sieboldianus | Fiveleaf Aralia | sun to shade | adaptable to most soils including dry |
| Fothergilla gardenii | dwarf fothergilla | sun to pt sh | moist, well drained, acid soil |
| Hydrangea quercifolia | Oakleaf hydrangea | sun to pt sh | moist, fertile, well drained soil |
| Ilex verticillata | Winterberry | sun to pt sh | moist, organic, acid soil; tolerates clay soil |
| Rosa rugosa | Rose | sun | organic, well drained soil; tolerates clay |
| Sambucus canadensis or S. nigra | Elder | sun | moist soil; tolerates dry |
| Sorbaria sorbifolia | Ural falsespirea | sun to pt sh | moist, organic, well drained soil; adaptable to wet |
| Staphylea trifolia | American bladdernut | sun to shade | moist, well drained soil, adaptable to wet |
| Stephanandra incisa | Cutleaf Stephanandra | sun to pt sh | moist, well drained soil; adaptable to wet |
| Symphoricarpos x doorenbosii | Snowberry | sun to shade | adaptable to most soil types |
| Xanthorhiza simplicissima | yellowroot | sun to shade | moist, well drained, acid soil; adaptable to dry and clay soils |
| Indigofera kirilowii | Kirilow Indigo | sun | tolerates dry soil |
The two bulbs that would fit on such a hill are what I call evergreen bulbs. They are Muscari and Ipheion, spring starflower. They bloom at the same time, look great together with, and have persistent winter foliage. Since they are bulbs, they like dry, stony areas.
With the plants from these lists, lovely combinations could be made of plants that flower together or follow one another so that there is something blooming all through the seasons.