Just because the greenhouse is plastic doesn't mean the quality to the plants is any lower.

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So long as you make sure there is ample ventilation inside the plastic greenhouse, your plants will thrive just as well as they would a glass greenhouse. If your plastic greenhouse came equipped with only side ventilation, you should take an extra step to make sure your plants are getting plenty of fresh air.

All you have to do is install a fan. In the winter time, the fan will make sure the warm air is circulated throughout the greenhouse. In the summer, it will allow the plants to have cooler air blowing on them. Whatever you do, if you grow shade loving plants, don't use anything that has a linseed oil base. The problem with using a linseed oil base inside a plastic greenhouse; is the plastic will absorb the oil. Once the linseed oil is absorbed into the plastic, you'll never be able to get it out of the plastic.