There are some things that you just can't grow in a greenhouse, no matter how hard you try. Here are some of the most common: 1. Fruits and vegetables that need a lot of sunlight. Tomatoes, for example, need at least eight hours of direct sunlight per day to produce fruit. If they don't get enough sunlight, they'll just produce leaves. 2. Plants that need very high temperatures. Some tropical plants will just never thrive in a greenhouse, because the temperature is too cool for them. 3. Plants that need very low temperatures. Conversely, some plants can't tolerate the warm, humid conditions found in many greenhouses. They'll either wilt or their leaves will turn brown and drop off. 4. Plants that need a lot of space. Some plants, like trees, just get too big for greenhouses. They'll outgrow their space and start crowding out other plants. 5. Plants that are sensitive to light levels. Greenhouses can be very bright places, and some plants just can't handle that much light. Their leaves will burn or they'll stop growing altogether.