Those of you who know me well, know that if I had to concentrate on just one area of horticulture, it would be that of herbaceous perennials. Among the many topics that could be considered in discussing herbaceous perennials are their characteristics, bed and border design, color in the bed or border, site preparation, seasonal care, and propagation. Here I will discuss site preparation.
There will never be as good an opportunity to tailor the soil to herbaceous perennials needs as the first time it is prepared. Try to start the preparation of the soil long before planting the bed or border. Start in spring for a fall planting and in fall for a spring planting. This will allow plenty of time for any organic soil amendments and/or pH modifications to take effect. To prepare the site for planting herbaceous perennials:
Clear all large debris from site.
Kill and/or remove all existing vegetation from the site for composting or disposal. Roundup, being non-selective and environmentally bio-rational, is an excellent herbicide to use to kill the vegetation. It might be necessary to retreat the site to kill persistent perennials weeds.
Dig an 18-to-24 inch-deep hole in the proposed site to study the soil profile and do a percolation test. Examine the depth of the topsoil and the character of the subsoil. Fill the hole with water. If the water takes more than 24 hrs. to drain out of the hole, the bed may have to be tiled to facilitate drainage.
Plant a green manure crop if it is possible to start soil preparation early. Sow winter rye if starting soil preparation in the fall for a spring planting. The densely-spaced stalks will choke out weeds, and tilling the winter rye into the soil in spring will add humus to the soil. Sow buckwheat, oats, or millet in spring and till them under in fall for a fall-planted herbaceous perennial border.
Spread 2 to 3 inches of organic matter over the surface of the soil prior to beginning to work it. This is equivalent to 3 to 4 yd.3/1000 ft. Spread and incorporate 1/2 to 1 yd.3 of organic matter/1000 ft2 in subsequent years. Compost ground corn cob, rotted hay, well-rotted sawdust, or sphagnum peat moss are just a few good forms of organic matter. Rototill or spade the organic matter into the soil to a depth of 8 to 10, if not even, 12 inches. Add no more organic matter than 1/3 of the final amended soil volume.
Consider using commercial composts. Commercial composts are an especially desirable form of organic matter to work into the soil while preparing it for herbaceous perennials as they are manufactured from materials that would otherwise end up in landfills or other waste disposal systems. Commercial composts may be composted sewage sludge (CSS), composted municipal yard waste (CMYW) such as leaves and grass clippings, composted garbage (CG) or composted paper mill sludge (CPMS). Again, incorporate 3 to 4 yd.3 of commercial compost/1000 ft of bed or border space.
Use care in amending soils with commercial composts. Most commercial composts, especially lime-dewatered CSS, have pH’s above 7.0. This admonition is especially important when amending a soil in which ericaceous plants, such as azalea, leucothoe, pieris, and rhododendron that require pH’s of 4.5 to 6.0, are to be grown.
Lime-dewatered CSS (such as Compro) has a pH as high as 8.2 by itself and a Compro-amended soil may have a pH as high as 9.4. Polymer-dewatered CSS (such as Earthlife) has a pH of 6.2 to 7.2, CMYW (such as Leafgro) a pH of 7 to 7.2, and CPMS (such as Glatcolite) a pH of 6.8 to 7.2.
Add 3 to 4 lb. of ferrous sulfate, FeSO4/1000 ft of bed or border space for each 0.5 pH point by which the amended soil needs to be dropped, down to a pH level of 6.0 regardless of the type of commercial compost used. It is necessary to use FeSO4 to lower the pH of amended soils to a pH of 6.0, because the micro-organisms that attack elemental sulfur, S, to convert it to the SO4 = ion, are not active above pH’s of 6.0. When FeSO4 is added to the amended soil it immediately goes into solution, freeing the SO4 = ion to join with the H+ ions of water, forming sulfuric acid, H2SO4 with a resultant rapid reduction of soil pH. Once the soil pH has been lowered to a pH of 6.0, it is possible to add elemental sulfur, as the micro-organisms that convert elemental sulfur to SO4 = ion are active. Compro would seem to be the commercial compost of choice to add to soils that are extremely acid and are not to be planted with ericaceous plants, but it would be better to add another commercial compost to those soils in which ericaceous plants are to be grown to reduce the amount FeSO4 that would need to be added to drop the pH to a desirable level.
Incorporate the organic matter, whatever form to a depth of 8 to 10, if not even, 12 inches deep. Do not work the soil when it is wet. Try not to step on the bed once it is tilled.
Consider double digging the herbaceous perennial bed or border. Double digging is difficult and time consuming, but well worth the effort.
Test the soil for soil type, pH, and nutritional status. This is done after incorporating the organic matter, because adding organic matter changes the soil type and the soil type dictates the amount of pH modifying agents that need to be added to effect the desired change.
Add and incorporate pH modifying agents and nutrients as dictated by the soil test. While it is not recommended to add pH modifying agents and/or nutrients without testing the soil, it is generally safe to add 5 lb. of ground limestone and 5 lb. of 5-10-5 or 5-10-10 fertilizer/100 ft2 of bed or border space.
If it is desired to garden organically, incorporate an organic fertilizer such as cottonseed meal, blood meal, or tankage to supply nitrogen and other elements. In lieu of chemical fertilizers use bone meal and rock phosphate for phosphorus, and greensand, wood ashes, and granite dust for potassium. Seaweed and fish oil provide trace elements in addition to nitrogen, phosphorous, and potassium.
Sterilize the soil if possible with a material (such as vapam) to reduce insects, soil borne diseases, and weeds.
Spread an organic mulch over the prepared bed or border. It is easier and tidier to spread the mulch before planting. Good mulches include compost, grass clippings, cocoa bean hulls, or bark chips. Mulches moderate soil temperatures, reduce soil water loss, help control weeds, reduce heaving of plants in spring, may keep plants from resuming growth too early in spring, will prolong root growth in fall, are attractive, prevent mud-splashing of the foliage and flowers, and add nutrients as they decompose.
Edge the bed or border in some fashion. This may be as simple as edging the sod with a sharp tool; implanting an edge of pressure treated wood, plastic steel, hard rubber, or aluminum; or partially burying bricks, stone slabs, or slate. An edging will be attractive and will help reduce the encroachment of the lawn.