Ask an avid gardener one thing they'd like to have to help them grow beautiful plants and she would probably say a greenhouse. A greenhouse gives the grower control over many aspects of plant growing, including extending the grower's season, allowing the grower to regulate sunlight, heating and cooling. Once you've had the advantages of a greenhouse, it's hard to go back to not having one for planting seeds that you watch growing until they're ready for the ground, and for overwintering plants in a sheltered place when winter comes.
Providing the Basics
Control the direct sunlight that your plants receive. If you can see shadows in your greenhouse, your plants are receiving too much direct light, which causes leggy seedlings. There are many types of shading sheets that you can purchase, but using flat sheets from your linen closet works fine. Depending on how your greenhouse is constructed and how large it is, you could hang the sheets inside from the corners or drape them over your greenhouse. This also helps regulate the temperatures inside the greenhouse.
Regulate temperatures as much as possible by opening doors and vents during the day. Some greenhouse have automatic vents that open and close based on temperatures inside the greenhouse but they aren't a necessity. When the temperatures are soaring in your greenhouse, use more shading sheets and as many fans as you can manage. Cooling is a much bigger problem than heating is for most greenhouse owners.
In a small greenhouse, a grower can use small ceramic heaters to maintain temperatures in the early spring and late fall, if needed. Overwintering most plants means they will endure the cold temperatures but without the weight of the snow and the cold winds. Having mass, such a stones, cement pavers and bricks provide a natural way to maintain temperatures, since they hold warmth for many hours.
Water your plants only as needed. As a general rule, it is better to under-water than to over water. Giving too much water to seedlings and plants can prove to be irresistible to molds and mildews which can be deadly to your plants. You can always provide a bit more water to a plant, but if you've over-watered there is no way to remove it. Using a hose with a sprayer keeps the humidity high but it can be difficult to judge how much of the water got to the plant. Using a watering can may take longer, but it is easier to see if a plant actually needs water or not.
Provide the right nutrients at the right time. All fertilizers are not the same and knowing what to give and when will make a big difference. Soils are often used improperly which affects growth as well.
When starting seeds, use a soil especially created for providing the right nutrients to ensure emerging seedlings will get what they need to grow at their full potential. When the time comes to transplant the seedlings into pots, use a potting soil that contains nitrogen (N), phosphorus (P), potassium (K) and vermiculite for good drainage and aeration. This type of soil stays fairly loose, allowing the roots good opportunity to grow. The front of the container will display a 3-part number, such as 10-5-10; this signifies the percentage of nitrogen (10%), phosphorus (5%) and potassium (10%). A decent percentage mix for potted plants is 10-10-10.
Nitrogen is likely used up more quickly than the phosphorus and potassium, so your mid-season fertilizer should contain a larger percentage of nitrogen. If plants are turning light green it is an indication that it needs nitrogen.
Pinch back seedlings to encourage bigger, healthier plants. Pick the tops of leggy plants by pinching the top between the thumb and index finger. This encourages the plant to grow fuller by growing more branches lower on the plant.
Spray plants with pyrethrum spray at the first sign of flying or crawling insects. It is easier to maintain an environment that bugs don't like than to get rid of them, so keep a garbage can with a heavy duty bag beside you when your are pinching plants back or removing dead leaves. Tossing them on the floor encourages decay, so throw them away in the garbage can instead.
Pyrethrum spray in an organic substance made from the flower of a pyrethrum, but it is very effective for most plant pests, including ants and aphids.