Some annuals, such as snapdragons and pot marigolds (Calendula), will tolerate frosts. These are called hardy annuals. But freezing temperatures kill most of the others. These are called tender annuals. Half-hardy annuals will tolerate mild frost.
If you are growing tender annuals, pay special attention to the average date of your last spring frost and don't plant them before then. Since many tender annuals only thrive in warm soils and air temperatures, planting them too early only stresses the plants.
The easiest way to get started with annuals is to buy greenhouse-raised seedlings at garden centers or other outlets. Usually, the plants are more robust and healthier than those we start from seed on our windowsills. They transplant easily and bloom early. Your choice of what to grow, however, is limited by what's popular. If you want to grow anything out of the ordinary, seek out garden centers that specialize in the unusual or start plants yourself from seed.
Look for healthy plants, but remember that what you are really buying is the roots. A damaged leaf or dying flower is not necessarily a sign that the plant is diseased. Look instead for a strong root system that fills out the pot or cell, but at the same time isn't overly root-bound.
Annuals that are in flower when you purchase them aren't necessarily better than those that aren't. It helps buyers know what the flowers look like, but it could also be a sign that the plants are over-mature and the roots may be pot-bound. If there is no picture or description on the label of a flowerless plant, ask the garden center staff for more information.
Also ask the staff if the plants have been "hardened off." Hardening off is the gradual transition that young plants need from their relatively soft life in the greenhouse to the rigors of surviving in the garden. If they haven't been so acclimated, you will need to gradually increase their exposure to sun, wind, and cool temperatures and reduce the amount of water they receive for a week or two before transplanting them. (More on transplanting below.)
Starting plants indoors
Growing your own plants from seed indoors can be rewarding -- especially if you want to grow species or varieties that aren't readily available as seedlings. But be forewarned, it takes some effort and attention to detail.
Without enough light, plants you start yourself will grow "leggy" -- long, weak and spindly -- as they stretch for light. For best results, you need at minimum a bright, unobstructed south-facing window and/or fluorescent grow-lights.
Start seeds in a sterile, light, free-draining seed-starting mix (available at most garden centers). Do not use garden soil. Young seedlings are especially vulnerable. Even good soil doesn't drain as well as seed-starting mix, and it may carry diseases that kill young plants. Select a time to start based on your expected last spring frost and when you plan to transplant the seedlings outside.
Use fresh seed, and determine whether or not they require light for germination. (Don't cover such seed with soil.) Plant in flats, pots, or cell packs at the specified depth. Covering seed with vermiculite instead of the seed-starting mix sometimes improves germination. Cover containers with plastic to seal in moisture and place out of direct sun in a warm spot until seeds germinate. Temperatures between 60 degrees F and 75 degrees F are usually adequate, but some seeds germinate faster and stronger at warmer temperatures.
When seeds begin to germinate, remove plastic and place containers in light. Keep soil moist so plants don't dry out. When seedlings develop two true leaves, thin to one per pot or cell. In flats, thin to about one plant every 1.5 inches, or transplant to individual pots or cells. Most potting mixes have very low fertility, so water with a weak solution of fertilizer. Harden off plants before transplanting.
Most annuals (those that are killed by frost and need warm soil and air to thrive) should be transplanted after the last average frost date. Don't be in a rush to get them outside if the long-term forecast looks threatening. Make sure the plants have been hardened off properly and are ready for the rigors of the garden.
If at all possible, choose a still, cool, cloudy day to transplant. Sun, heat, and wind can stress seedlings, causing leaves to wilt and even killing the transplants. Waiting until late in the day on a sunny day will at least give the transplants over night to start getting acclimated.
Before transplanting, remove any weeds that may have cropped up in your previously prepared planting bed. Loosen the soil where you will transplant the seedling and dig a hole large enough to accommodate the root system.
If the roots are pot-bound -- closely pressed against the side of the container and growing in circles around the inside -- tease them out so they will start growing into the surrounding soil. If you don't, they may keep growing in circles and the seedling will remain stunted. In severe cases, you may need to use a sharp knife to cut the circling roots, to stimulate new root growth. Pinch back any flowers and flower buds that have formed on the plant and perhaps a little of the foliage to compensate for the root damage.
It is important to plant most annuals at the same depth at which they were grown in containers. Planting too deep or too shallow stresses the plants. If seedlings are in peat-pots, peel away the upper edge of the pot so that the entire pot is below ground after planting. If any of the peat pot protrudes above the soil after planting, it can wick moisture away from the seedling.
Water plants thoroughly after transplanting. Don't let the soil dry out for the first week or two while the seedlings' roots get established. Check daily to make sure that the soil around the transplants is moist below the surface and that the plants aren't wilted. Mulching can help soil retain moisture.
Be sure to space plants properly. Refer to the seed pack or label about specific spacing suggestions for the variety you are growing. (Use spread information in the Flower Growing Guides to help determine how far apart to space plants.) Those tiny annual seedlings often grow larger than you ever dreamed and get too crowded. On the other hand, if you want them to form a continuous border or clump, don't space them too far apart.
With some annuals, the seed is difficult to germinate or takes a long time to sprout. In these cases, you are better off buying plants or starting the seed yourself inside. Other species are easy to plant and grow right in the garden. They may start flowering several weeks later than they would have if started inside. But direct seeding these in the garden saves a lot of effort.
The biggest problem when direct seeding is that your garden soil may not be as inviting to seeds as seed-starting mix. If that's the case, make small furrows or holes, plant the seeds, and cover to the prescribed depth with wetted vermiculite or seed-starting mix. Keep soil moist until seeds sprout, then thin to proper spacings.
Many annuals self-seed prolifically if you don't remove flower heads before they mature. The seeds usually sprout the following year near where the plant grew (or wherever wind, water, or animals carried them). If you want plants in these locations, thin as if you planted them. If you don't want them there, hoe them out as if they were weeds.