We use Wall Of Water (see detailed review) and have several small greenhouses to grow the vegetables we want in our short growing season. Last year, I used a min-max thermometer to show how the greenhouses increased the total Growing Degree Days (GDD). This season, I have done the same to measure any GDD advantage offered by the Walls Of Water.
To measure the difference between the outside temperature and the environment inside the wall of water, I put a min-max thermometer inside the wall of water and another min-max thermometer in a protected, shady area under the deck. So far, I can compare the minimum and maximum temperatures for the first eight days after transplanting.
Tables 1 shows temperature comparison between inside the Wall of Water and the outside temperature. The outside minimum temperature ranged from 34 to 55°F while the minimum temperature inside the wall of water ranged from 45 to 60°F. Many warm weather vegetables do not start growing quickly until nighttime temperatures are above 60°F, but 45 to 60°F is much better than 34 to 55°F.
Table 1. Minimum and Maximum Temperatures Measured Outside and Inside Wall of Water
|Outside||Wall of Water|
|26 – May||71||42||78||52|
|27 – May||62||41||76||50|
|28 – May||60||39||75||48|
|29 – May||64||35||78||48|
|30 – May||68||35||78||48|
|31 – May||78||34||85||45|
|1 – June||85||43||85||50|
|2 – June||85||55||85||60|
Table 2 compares the Growing Degree Day (GDD) calculated for outside and inside Wall of Water. GDD calculation is a method for evaluating the effect of temperatures on plant growth or to compare the timing of one growing season to another. The growth and development of some plants are highly correlated with GDD. GDD is based on a minimum temperature because little plant growth occurs below that temperature.
For example, different varieties of tomatoes need between 1,300 and 1,900 total GDD to ripen and varieties of corn need between 1,600 and 2,700 GDD. So if you live in a 1600 GDD climate like we do, you can grow corn, tomatoes and other warm weather vegetables, but need to select the correct varieties and it doesn’t hurt to do something to boost total GDD.
Growing Degree Day (GDD) Calculation
GDD = (Tmax +Tmin)/2 – Tbase
- Tmax = Maximum Temperature
- Tmin= Minimum Temperature
- Tbase= Baseline Temperature
50°F is a common base temperature used for tomatoes, peppers, eggplant and cantaloupe. Many sources also limit upper temperatures to 85 or 95°F, because little growth occurs when plants are heat stressed or because fruit set is not possible at high temperatures. For these tables, I used 85°F as a maximum temperature, so as not to overestimate the importance of maximum temperature. Also, official temperatures are measured in the shade and that is not applicable inside the wall of water.
From Table 2, you can see that outside the wall of water, four of the first eight days would have allowed little plant growth as calculated GDD was 1.5 or less. Two of the days were even negative, which basically means that plants could not grow. Negative GDD days do not subtract from the cumulative GDD value, but are treated as zero. After eight days in the wall of water, my plants have an 120.2 – 49.5 total GDD advantage over plants that are outside of the wall of water.
Table 2. Growing Degree Day (GDD) Comparison for Outside and Inside Wall of Water
|Growing Degree Days (50)|
|26 – May||6.5||15.0||6.5||15.0|
|27 – May||1.5||13.0||8.0||28.0|
|28 – May||-0.5||11.5||8.0||39.5|
|29 – May||-0.5||13.0||8.0||52.5|
|30 – May||1.5||13.0||9.5||65.5|
|31 – May||6.0||15.0||15.5||80.5|
|1 – June||14.0||17.3||29.5||97.8|
|2 – June||20.0||22.4||49.5||120.2|
The GDD advantage is shown best in Figure 1, which shows Cumulative Growing Degree Days between outside (GDD), inside the Wall of Water (WoW GDD) and the average for our area (Avg GDD). So for the first eight days since planting, GDD is been behind the average GDD for the same days by about two days. The GDD inside the wall of water is about six days ahead of schedule or about about eight days ahead of plants that are outside.
Figure 1. Cumulative Growing Degree Day (GDD) Comparison for Outside, Inside Wall of Water and Outside Average
Increase tomato harvest with Wall Of Water
With our short growing season, an eight day head start could be the difference between a decent harvest and no harvest at all. My first attempt at growing tomatoes in this climate (without Wall O Water), ended with a killing freeze the first night after I noticed the first ripe tomato. Enough to make a grown man cry.
We have had better success with tomatoes using the Wall O Water than in our greenhouses. The GGD is boosted earliest in the season when we need it most, but the high temperatures inside the greenhouse make it difficult for fruit to set. So we harvest the most tomatoes using Wall O Water early in the season, then cover the entire plant with a greenhouse tent late in the season.