First, select a site that has good drainage and provides the amount of sun or shade needed. Prepare a bed by removing all weeds either with herbicides or pulling and then working up the soil at least 6” deep. Raising the bed 2-4 inches above the original ground level will be helpful with drainage. Beds that provide good drainage are important as many ground covers do not do well with “wet feet,” and the poor drainage will cause rotting. After the bed is prepared, you’re ready for either transplanting seedlings that were started indoors or directly sowing the ground cover seed on the bed.
General suggestions for starting seeds in doors for spring transplanting: Sow seeds in late winter, 8 to 10 weeks before planting outdoors in the spring (after danger of any frost). Use shallow trays filled to the top with moist potting soil. Sow seed in shallow rows and cover with soil only to the thickness of the seed. Bottom water, and do not let the seed dry out. Keep trays at a constant temperature. As seedlings develop leaves, transplant into larger pots. An application of liquid fertilizer is recommended for seedlings, using a light spray of the fertilizer solution. Before transplanting into the bed, harden the plants for 3 days in a protected area outside. Transplant, use a light application of liquid fertilizer or starter fertilizer and water well.
General suggestions for directly sowing seeds in bed: Once the bed is prepared, spread a 1-2” layer of potting soil over the bed. Many of the seeds for ground cover are very tiny, and it’s helpful to mix the seeds with very fine sand. This gives you more matter to broadcast spread over the desired area. If you need to sow between stones, using a salt shaker is a good way of controlling where the seed/sand mixture goes, and it allows you to get into the small crevices. Use a light dusting of the potting soil or peat moss over the top of the seed, but do not cover more than the thickness of the seed. Moisten the bed with a fine spray mist and maintain the moisture until the seed has sprouted. It’s recommended to weed the bed as the seedlings grow as well as using a liquid fertilizer.
If your only seed starting experience has been with easily germinated vegetables or annual flowers, more patience is going to be required when it comes to growing perennials from seed successfully. Some types germinate within days, others take several weeks, and a large number of perennials require what is called stratification -- basically, simulating the conditions that exist outside over the winter. These types of seed are sometimes described as cold germinators. The usual trick is to place the seed with some moist, sterilized commercial seeding mix inside a plastic bag, then storing it in a refrigerator for a period of time to break down the natural chemical germination inhibitors within the seed. A typical period of time is about three to four months. Then the seed is sowed as usual and started indoors under lights. Another approach is to sow the seed in late fall in pots, then leave it outside in a protected (but unheated) cold-frame for the winter.
Many annuals are easy to seed directly into garden soil. Others are best started indoors under lights in late winter or early spring. Generally speaking, annuals fall into three main categories, which determine when and where you should sow their seeds. For all categories, a good rule of thumb is to plant seeds at a depth of two or three times their diameter.
Some annuals are so good at fulfilling their mission in life—flowering and setting seeds—that they will self-sow readily under the right conditions and produce brand-new plants the following year. Common annuals that can self-sow vigorously include ageratum, petunia, foxglove, annual larkspur, forget-me-not, calendula and wild or striped mallow (Malva sylvestris).
Hardy annuals: Can be direct-sown in the garden as early in the spring as the soil can be worked. For an earlier start, sow them indoors in flats eight to ten weeks before the last spring frost date, and transplant them to the garden about a month later, after hardening them off. Some hardy annuals can also be direct-seeded in the fall, and these plants will flower much earlier than plants seeded in the spring. When fall seeding, you can plant the seeds a bit deeper than you would in the spring, and spread some mulch over the seedbed after the ground has frozen. Examples of hardy annuals include bachelor's-buttons, calendula, spider flower (Cleome hasslerana), pinks (Dianthus spp.), larkspur, linaria, Shirley poppies (Papaver rhoeas), nigella (love-in-a-mist), scabiosa (pincushion flower), snapdragons, lavatera, annual baby's-breath (Gypsophila elegans), heliotrope, stocks and sweet peas.
Half-hardy annuals: These can be direct-sown outdoors after the threat of hard frost (temperatures below 25°F) is past. Indoors, start seeds in flats six to eight weeks before the last spring frost date, and harden off the plants before transplanting them to the garden. Once they have hardened off, half-hardy annuals can withstand a light frost. Examples include statice, nicotiana, painted-tongue (Salpiglossis sinuata), China aster (Callistephus chinensis), and various types of salvias and chrysanthemums.
Tender annuals: Seed can be sown directly in the garden only after all danger of frost is past. For an earlier start, sow seed indoors four to six weeks before the last spring frost date for your area. Examples include marigolds, morning glories, zinnias, sunflowers and tithonia (Mexican sunflower), cosmos, amaranth, ageratum, celosia and gomphrena (globe amaranth).
Site Preparation Steps:
1. Determine your soil type. Each soil type will require preparation, but sandy and clay soils may require additional care and steps.
Sandy soils dries out quickly, may be low in nutrients and have a low pH. Be prepared to add supplemental water to germinate your wildflower seed unless you are in the rainy season. Adding additional organic matter, such as composted yard wastes, will increase the soil's water holding capacity as well as available nutrients for you wild flowers. If you do not choose to add organic matter, a soil test will help you determine whether adding a low-nitrogen fertilizer and lime will be necessary.
Loamy soil contains more organic matter than both sandy soil or clay soil and therefore retain adequate moisture while providing necessary drainage. These soils are the easiest to prepare for wildflower seeding and provide an excellent wild flower site.
Clay soil is heavier than sandy or loamy soils. It retains water easily, but does not allow proper drainage for most wildflowers. If it dries out during a hot summer, it becomes hard and prevents wild flower roots from penetrating deeply into the soil. Clay soils generally contain sufficient nutrients, but benefit from added organic matter to increase drainage. Another method of adding organic matter to the soil to improve wildflower plantings is to plant a green manure crop such as buckwheat or winter wheat. Plow this crop under while actively growing to incorporate the roots, stems and leaves into the soil. As they break down they will enrich the soil and add organic matter to help your wildflowers grow.
2. Remove existing vegetation. Reducing competition for space, light, moisture and soil nutrients is essential to the success of your wildflowers. This can be done by smothering, mechanical removal or by using herbicides.
Smothering: vegetation on small areas can be effectively killed, along with dormant seeds in the top several inches of soil, by covering the area with black plastic. The increased soil temperature will kill weed seeds, while the lack of sunlight and moisture will kill existing vegetation. For the full effect, leave the plastic in place for a full growing season. Cultivation: using this technique alone will require repeated passes, at 2-3 week intervals, preferable with supplemental water, over a full season to be effective. This is because tilling will bring to the surface more dormant weed seeds which will interfere with your wild flower planting. Herbicides: using herbicides such as Round-up will usually require two applications, 3-4 weeks apart to kill the existing vegetation. After the first application, wait for the weeds to die back then remove them by cutting, weed whipping and raking. You may want to shallow till the ground before you plant your wildflowers.
Because wildflower seed mixtures contain seeds of vastly different sizes, blending an inert carrier like fine sand or vermiculite with the seed will help to insure an even distribution (4:1 sand to seed is recommended). For small areas, hand broadcasting the wild flower seed along with an inert carrier will work quite well. For larger areas, mechanical seeding such as a cyclone-type seeder, a brillion seeder or a no-till seed drill work well, but tests must be made in order to achieve the proper planting rate of the wildlfower seed with each type of machine. Once the seed has been planted, it must be covered to a maximum depth of 1/8 to 1/4 of an inch. This can be achieved by lightly raking the seed in with a hand rake for small areas, or by using a drag mat behind a tractor for larger areas. If a drill seeder is used, firm the soil after drilling with a cultipacker to insure proper seed/soil contact. Care must be taken not to cover the wildflower seed too deeply as this is one of the primary reasons wild flower plantings fail.
Weeds left uncontrolled will quickly take over any wildflower planting. Removal of weeds during the initial site preparation must be followed. Once germination of the wildflower species has begun, newly appearing weed should be removed. Overseeding wildflowers with the originally planted mix or an all-annuals mix in the fall or early spring will help to fill in the bare spots and keep weeds down. The newly seeded wildflowers will increase wildflower density to crowd weeds out.
Post Planting Care:
The planting must be kept moist 4-6 weeks, using supplemental water if necessary in order to ensure germination of as many wildflower species as possible. As the planting becomes established, water can be gradually reduced from the wild flowers. Many wildflowers are drought tolerant once established, but must receive adequate water in order to germinate and to become established. You should begin to see wildflower seedlings within 2 weeks and the first flowers in 6-8 weeks. As the season progresses, several waves of color will be seen according to the blooming time of the wild flower species. In dry climates or in drought condition, 1/2 inch of water per week will lengthen the blooming period of the wildflower display.
Once the wildflower planting has stopped flowering and set seed, it can be mowed to a height of 4 - 6 inches. In most climates, this will be in mid-October. Mowing will help to scatter the ripe seeds for next year's blooming. After fall mowing or early the following spring, consider supplementing the site with some new seed in order to more firmly establish the permanence of the planting and to suppress weed growth.