by Gena Moore, CFSA’s Organic Research Coordinator
I’m sure you’ve heard that no two snowflakes are the alike. The same can be said for high tunnels. The fact that all high tunnels are different makes determining planting dates difficult for many growers. Accepting general planting date recommendations can be a risk when working in high tunnels and risk taking in agriculture is, well, risky. Although determining planting dates for high tunnel production can be challenging, if you get to know your tunnel, you can minimize your risk.
First, let’s define what a high tunnel is and what makes each one unique. A high tunnel is a framed structure with plastic covering that is managed through passive ventilation. The structures are usually metal but some are made of PVC pipe or wood. Passive ventilation is achieved through roll up sides, vents, and end doors. Now, if most tunnels are made of similar materials, why is each one so different? The factors that make each tunnel unique are:
1. Local Climate and Geography
2. Construction
3. Management
To discuss these factors further:
- Your local climate and weather patterns will affect your high tunnel. Also, the amount of sun and shade your tunnel receives drastically affects the air and soil temperature. Answer these questions: What hardiness zone are you in? When are your estimated frost dates? Generally, what weather conditions can you expect during each season? By understanding your local climate, you have a starting point when trying to understand your high tunnel.
- Most high tunnels are constructed with either one or two layers of 6ml plastic. The quality and composition of the plastic covering will affect your tunnel’s conditions. The positioning of your tunnel in relation to the sun will affect the amount and timing of sun exposure, which in turn affects the air and soil temperature. Also, the size and shape of your tunnel will impact its ability to accumulate and retain heat. Finally, ventilation will determine the air flow which impacts several aspects of high tunnel conditions including air temperature and humidity.
- Management is the most influential factor that makes all high tunnel conditions unique. Some of you, like me, may use black plastic mulch and row covers religiously. Whenever you add something like row cover or mulch, you increase the temperature of air and/or soil even more than the high tunnel itself can provide. Venting and closing up the tunnel at the most appropriate times also effects the tunnels conditions. I’m around my tunnel all day and can adjust the venting at any time but many people do not have this option.
Now, down to what everyone wants to know: When should you plant in your high tunnel? The answer depends on the above factors which create the conditions in your tunnel. Study your high tunnel. Know what your tunnel is made of and how that impacts your air/soil temperatures. Some great tools are hi/low air thermometers, humidity probes, and soil temperature probes. Also, get to know your local climate. There are countless resources on the internet that summarize local climates, try http://www.ncdc.noaa.gov/. Finally, decide on how you will manage your crops. Using row cover will enable you to plant earlier but requires more labor. Planting one crop per tunnel makes management and planning easier, but multi crop and successional planting are achievable as well.
Start with some safe planting dates, about 3-4 weeks prior to typical field planting dates. Keep track of the conditions in your tunnel. Get ready to make more educated decisions in the following seasons by matching your specific high tunnel conditions with the optimal growing needs of your targeted crops.
If you want help planning crops, maximizing your production potential or solving high tunnel issues, apply for CFSA’s High Tunnel Consulting! It’s free for CFSA members.