I have a 1,300-sq.-ft. dome greenhouse. Our cucumbers and squash have huge leaves and a ton of blooms, but the fruit does not develop very fast. Any possible solutions? I use a flood and drain system.

When it comes to large-fruiting plants like cucumbers and squash, there are number of possible reasons why the fruits are slow to mature. The first thing to consider is air temperature. The optimal daytime temperature for rapid growth is between 75 and 80°F. Prolonged daytime temperatures above this are tolerable, but may negatively affect the fruit. Optimal nighttime temperatures should not dip below 65°F, as this could stunt plant and fruit growth.

The second thing to consider is water temperature. The optimal water temperature is between 68-72°F for just about any type of plant. Above 80°F will begin to cause heat stress, and above 85°F, the plant will want to go into survival mode and abort fruiting. Water temperatures below 60°F will also stunt plant growth.

The next thing to consider is humidity. Even though you’re growing in a greenhouse, it is worth mentioning just in case that low humidity can slow down fruit growth as well as lead to small, undersized fruit, especially for large fruit-bearing plants like cucumbers, tomatoes, squash, peppers and melons. Humidity levels should be around 80-85%, or at least range between 70 and 90%.

Along with good humidity levels, you need to have good air circulation. Without good air circulation, moisture can build up and pool on the foliage, causing disease and fungal spores to germinate. Most people in your shoes choose to lower their humidity levels when they should be increasing their air circulation instead.

Other things to look at are your nutrient levels, your water levels and your water-cycling schedules. Cucumbers and squash are relativity heavy feeders, so your reservoir’s size, and how often you do nutrient changes, is important. The general rule of thumb for minimal water volume for large plants is 2.5 gal. of water/nutrient solution per plant. However, there are many variables, and I would recommend you double that to 5 gal. per plant.

Large plants don’t like having wet feet in the root zone. They prefer even moisture. It’s better to have shorter, more frequent watering cycles than longer, further apart cycles. You only need to keep the water on long enough to get the growing media wet, and you want it to go on often enough to keep the growing media moist throughout the day. As the plants get bigger, you may need to shorten the time between watering cycles as the plants suck up more and more water.

Make sure to replace the water the plants drink with fresh water from time to time (in between nutrient solutions changes) to maintain the same water volume in the reservoir. Water-level fluctuations cause fluctuations in nutrient concentrations, which stress out the plants. The more the water level fluctuates, the more the nutrient levels will as well. Also make sure to change your nutrient solution regularly or your nutrients can become unbalanced or be depleted. This is even more important with heavy-feeding plants like cucumbers and squash, especially as they get bigger. How often you need to change your reservoir depends on many factors, so there are no hard and fast guidelines for this.

The final consideration is light levels. Fruiting plants need more light than non-fruiting plants and plants that aren’t fruiting yet. Once the fruit starts growing, the energy load on the plant is greater, so the light levels that were fine for growing foliage might not be enough once fruiting begins. Since you are growing in a greenhouse, I assume you’re relying on sunlight, but could it be that the plants are shaded by other plants, trees or other objects such as buildings? How much light is getting through the greenhouse glazing? Could it just be that it’s been too cloudy in your area?

Off the top of my head, and without knowing any particulars, these are my first thoughts, and I suspect the problem could be a combination of these factors. Good luck!