How to manage your greenhouse in the springtime

April 10, 2012 by  
Filed under Blog, Greenhouse

Tuesday 4-10-2012

Spring is actually a very tricky time to grow plants in a greenhouse. You really have to pay attention to the temperature, humidity, ventilation, watering needs, and pests.


Lean to greenhouse facing south (shed on backside)



It really depends on what you have growing in the greenhouse as to what temperature is ideal. For your warm weather plants such as tomatoes, peppers, and eggplant daytime temperatures in the 70’s and 80’s with nighttime temperatures in the 50’s and 60’s are ideal. For your cool season plants like lettuce, spinach, broccoli, and cauliflower, they prefer 50’s and 60’s during the day, and 40’s and 50’s at night. These plants can even tolerate mid-30’s at night, just don’t let them freeze. I try to keep it simple, so I have an electric heater that is attached to a thermostat, so it only kicks on when the temperatures in the greenhouse dip below 40. This keeps all my warm weather plants alive, and doesn’t waste too much electricity. I also have (2) 50 gallon black trash cans filled with water to help soak up the sun during the day and release the stored heat at night, lessening my electricity use, and adding valuable humidity.


During the springtime, you have to be very careful to watch the temperature levels. It can be 70 degrees and beautiful out, but it can be 90 plus inside your greenhouse. This can be life threatening to your plants, especially your cool season plants. My broccoli and cauliflower plants got burns on the tips of their leaves in just a couple of hours on a 70 degree day. At this point in the season with sunny 50-60 degree days, all my spinach and lettuce have been moved outside. I would also like to move my broccoli and cauliflower transplants outside to harden off, but it has been so windy (20 MPH), that I am afraid the root systems are not yet ready for that given that the greenhouse is wind free. I am waiting for a string of calmer days to put them out. My warm weather plants will stay in the greenhouse until the end of April or beginning of May.


Leafy greens put outside                                                              Broccoli burned leaf tips


Temperature gauge controlling power to the heater                        Electric heater



Most plants prefer humidity levels in the 50-60% range. This can be difficult to maintain. One way would be to install misters set to a humidistat. I have not done that to this point. I manage it the old fashioned way, by watering as needed. I do have a humidistat that I watch, but it is pretty obvious when it is too dry. In the winter when it is cold and cloudy out, it is easy to maintain a high humidity. Sometimes I can go every other or even every three days between watering. When it gets warm and sunny, I am not able to keep the humidity I want. During these times I make sure to give the plants a good soaking twice a day. My (2) 50 gallon trash cans filled with water help a little to keep the humidity up.


Seed starting                                                                                     Humidistat on the dial



When it gets really warm or if the air is stagnant, ventilation is a good idea. I have vents on top of my greenhouse that are filled with a gas that raises the vents as the greenhouse heats up during the day, and shuts them as they cool off. These vents are nice, but they are not enough in the spring. On a sunny day over 50 degrees, I also open the half door and the side vent to provide a cross breeze. Unfortunately, I have to shut these every evening. Installing fans is also a good idea, although at this time my greenhouse is not equipped with fans.     



It is necessary to have a water source in the greenhouse. It will get old really quickly if you are filling up watering cans. Also, if you are heating the greenhouse in the winter, you don’t have to worry about frozen water. I have a hose bib that I water the plants directly from. I use a nozzle that allows misting and gentle showers so as not to blow out delicate seedlings. In the spring when it is sunny I am watering twice a day, giving the plants a thorough soaking. If it is cloudy and rainy out and my soil in pots are still damp, I may not water at all.   

Water source                                                                   50 gallon can filled with water w/ bug screen



If you are using sterile potting soil, chances are your pest pressures will be minimal, but if you are using compost from your garden, you may end up bringing pests into the greenhouse. The problem with this is that their natural predators may not be present inside, so you may end up with a serious insect infestation. I do use compost from my garden, because I like my seedling to grow in the soil they will be transplanted into, and it is cheaper. So I monitor my plants closely for pests. If I can just put plants outside or pick the pests off by hand, I will do that. In some rare cases, I will spray an insecticide.   

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