Three options exist for starting annual plants. They are:
- direct seeding,
- indoor seeding and transplanting, and
- purchasing and transplanting bedding plants.
The simplest and cheapest propagation method is to plant seed directly into the garden site. A wider array of species and varieties are available as seed, providing the gardener with greater choice. The advantages of direct seeding are offset by the tendency for plants to be slow and erratic with respect to emergence and early growth. This may delay flowering and shorten the color display during the blooming period. This is especially true in the short growing season areas of Idaho’s mountainous regions.
Refer to the seed package for recommended date of planting. Plant seed where you would like the flowers to grow in the garden. Place seed in shallow trenches and cover lightly. Plant extra seed and thin after emergence, if necessary. Refer to the seed package again to determine seeding rate and depth. Maintain good moisture at the soil surface by misting lightly until the plants emerge. Once established, deeper irrigation should be applied after the top 1-2 in. of soil dries out.
Growing your own transplants has the best features of both direct seeding and purchase of transplants. It gives the cost advantage and plant choice of direct seeding while making it possible to get a head start on the growing season.
The key to success is providing appropriate conditions for germination and early growth of the new seedlings. Soil, temperature, light, and moisture are the most important elements. The soil medium must be free of disease organisms that may cause death of germinating plants. The best soil medium is a commercial potting soil. Containers may range from recycled plastic pots, paper cups, or commercial seed plug trays. Wash and disinfect all containers before using. Temperatures should be warm enough to allow germination (e.g. 60-75º F during the day and slightly cooler at night).
Unless a greenhouse is available, artificial lighting will be required. Even a south facing window does not supply young plants with enough light to keep them from getting ‘leggy’ and weak. Some seedlings may need as much as 18 hours of light to be healthy. Fancy equipment and expensive “grow lights” are not necessary. A standard fluorescent shop light, easily found at home improvement and hardware stores, fitted with one “warm,” and one “cool” tube works very well. Suspend the light fixture 12-18 inches from the plants and raise as growth occurs. It is also important to keep soil moisture balanced between too wet and too dry. This required frequent, light irrigations.
Another important process in producing healthy transplants is called hardening off. This refers to the procedure of adapting the plants to outdoor conditions to reduce transplant shock. This can be done by placing the plants outdoors in full sun for increasing amounts of time each day for a week or ten days prior to planting. Hardening the plants will improve survival and increase the early growth rate. Those fortunate enough to have a cold frame or unheated greenhouse can use these structures for this purpose as well as starting seeds.
Purchasing Bedding Plants
Buying partially-grown plants is the easiest and quickest way to establish annual flowers. It is also the most expensive and provides the least in the way of plant choice. But, transplants will result in quicker blooms and longer flowering periods.
When choosing transplants, it is best to buy from a nursery or garden store having personnel knowledgeable about local growing conditions. This will assure availability of adapted species and varieties. Do not look for the largest plants or necessarily, those in bloom. Seek plants with good dark green color, healthy root systems, and no sign of disease or pest problems. Try to find plants that have been hardened off, in order to aid the transition to the yard.
Another important consideration for growing transplants is the timing initial planting. To properly make this decision for a particular flower, it is essential to know the amount of time needed to produce a transplant and the approximate date of intended transplanting outside.
Whether transplants are purchased or self-grown, the process for placing them outside is the same. The first decision is deciding when to place the plants outdoors. Transplanting date is based on the date of last frost in a given area. Tender annuals should not be planted to the garden until 1 to 2 weeks after the average last frost date. It is, after all, an average and frost will commonly occur after the printed date.
To estimate the last frost date in your area, look at the Idaho chart compiled by Ed Hume Seeds.
Tonie Fitzgerald, from the Spokane, Washington County Extension office compiled a table listing appropriate dates for planting and/or transplanting common annual flowers. The dates should be reasonably accurate for much of northern and south-central Idaho. Dates for the Treasure Valley of southwestern Idaho will be 2-3 weeks earlier, southeastern Idaho a few days later, and the high country up to 2 weeks later.
It is best to transplant on a cool, cloudy day with little wind. This will allow acclimation under conditions of limited water loss. After removing a plant from its container, tease roots away from the surface of the root ball. Don’t plant the seedlings too deep. Bury the root ball in a hole sufficiently deep only to bring the soil slightly above the pot soil level. Space the plants according to the instruction on the seed packet or nursery pot label. For the first 7-10 days, water the plants frequently and lightly. Remember that early on the pot soil holds all of the roots and is the only source of water. The root ball will need to be wetted as often as it would in the pot until the roots can grow into the surrounding soil.