Starting your vegetable garden from seed can be a very rewarding endeavor as you watch and nurture the seed into a healthy plant. There are risks to you plant using this method because the baby plants are very fragile in the early stages, but it can be a cheaper way to go if you know what you're doing. Another way to get started is to buy already growing plants from a nursery.
If you choose to buy a plant from a nursery and put it in your garden, it reduces a lot of the work involved in making it healthy. However, sometimes incompetent nursery workers can ruin the future of the plant by putting certain chemicals or fertilizers on it. Learning to choose the healthiest plant available can make a difference in the outcome.
How nice does the plant you're considering look? You really can tell a lot by how a plant looks. It should show no signs of disease or pests. Holes in the leaves and wilted stems point to a plant that has bugs or was grown in poor soil.
While they may be pretty, any plant that is already flowering may be traumatized by replanting. It is better to find plants that only have buds. If flowering plants is your only choice, be prepared for the sake of your plant to cut off the flowers before replanting. The future health of the plant will be better, and reduces the risk of losing the plant in the process.
Always check the roots before purchasing the plant. Of course if the roots are in absolutely terrible condition you will be able to tell by looking at the rest of the plant. But if the roots are just slightly out of shape, then you probably won't be able to tell just by looking at it. Inspect the roots very closely for any signs of brownness, rottenness, or softness. The roots should always be a firm, perfectly well formed infrastructure that holds all the soil together. One can easily tell if the roots are before their prime or past it, depending on the root to soil ratio. If there is an excessive amount of roots with little soil, or a bunch of soil with few roots, you should not buy that plant.
It never hurts to ask the nursery employees about plants that you find abnormalities in. Irregular features in the leaves or the shape of the roots may indicate an unhealthy plant, but by asking a nursery worker you may find a satisfactory explanation for it's appearance.
Know what you're looking for, know how to choose, and you can be more confident that you will find healthy plants for your garden.