Unfortunately, without the proper vigilance, the same greenhouse that transforms my seedlings into green giants can mutate ordinary problems into cyclones of destruction, capable of wiping out the entire contents. It has happened to me more than once, so if you are new to greenhouse gardening, you may want to keep a keen eye on the following.

First, remember that when it comes to greenhouses, hotter is not always better. Each variety has its own optimum temperature preference, and sometimes these desires change with the stages of plant development. However, in general, the air temperature of a greenhouse should not rise above around 85 degrees. After that, your tomatoes and cukes may have trouble with pollination.

The problem with temperature is that it can rise devastatingly quickly. More than once I have left for a morning's errands with the greenhouse a comfortable 65 or 70, only to return at noon to find the thermometer registering 112 or higher. And leaving the door propped open is insufficient on especially warm or sunny days, for without cross ventilation the air hangs heavy, still and hot.

The bottom line is that you need a way for cool air to rush in and hot air to be driven out. This can be done with the installation of passive vents, enough openings that a sufficient volume of air can be exchanged so that temperatures are kept at a reasonable level. Automatic ventilation or fan systems let you conveniently preset the temperature at which vents will open or fans will leap to life but can be pricey to install.

I have a built-in fan with a timer set for the hour when I know from experience the full power of the sun will be focused on my plants. I leave the door of the greenhouse ajar, and when the fan (which is positioned close to the apex of the roof) comes on, it sucks the hot air out and allows room for the cooler air to come in through the door.

Second, keep an eye on humidity, for while it will not disable your plants as rapidly as heat will, saturating dampness can be permanently damaging. According to the Alaska Cooperative Extension tip sheet: "Controlling the Greenhouse Environment," by horticulture specialist Wayne Vandre, "The plant transpiration rate is affected by the relative humidity because it determines the vapor pressure difference between the leaf surface and the surrounding air." In other words, if it is too damp the plants cannot pass the oxygen and water they produce back out into the atmosphere and growth can be stunted.

The tip sheet goes on to say, "A relative humidity between 25 and 80 percent will not adversely affect most plant growth. A higher rate can restrict transpiration and it also may contribute to disease problems." So, the same high humidity that stresses your plants provides the perfect conditions for disease growth--two reasons for keeping it under control.

Watering only when absolutely necessary helps, as will making sure you don't leave pools of water lying around when you drag the hose back out the door. The same venting that reduces temperatures will also reduce humidity.

Third, air circulation is critical, and ever more difficult to ensure as plants begin reaching for each other and the ceiling. When the air is stagnant, humidity builds up, diseases thrive, and plant stems don't become as strong as they do when air movement is more vigorous. Fans and venting will help maintain a healthy circulation (see a pattern here?).

Trellising sprawling plants, or trimming some leaves will also encourage circulation. In severe over-crowding, create a few strategic holes in the jungle by cutting down a few plants. (Never pull, as you disturb the roots of other plants that have entwined themselves with the chosen sacrifice.) And next year remember that while things look impossibly lonely in May, they won't in July.

Fourth, be a fanatic about pest control. I am not exaggerating when I say that on a daily basis I spot-check buds, the undersides of leaves and the places where stems and stalks intersect. At the first sign of aphids, I squash individual offenders or wash the entire plant. I also unleash a legion of ladybugs at least once mid-season. If a plant looks to have caught a disease, I am ruthless about culling. I first try cutting off just the affected parts, but if the spots, splotches or rot continues I decapitate at root level.

Finally, keep your plants as healthy as possible, so that they can survive the occasional soaring temperature or opportunistic pest. This means not allowing your plants to starve for water or nutrients, watering as close as possible to the soil line rather than overhead, not exposing them to pathogens like hands tainted with nicotine and keeping living conditions sanitary.

It seems like a lot of work, but the reality is that weekly attention will keep minor problems from turning into the plague.