THE OCCURRENCE OF FROST 



353 



afternoon and 3.4° cooler in the morning. Below the surface, however, 

 the sod loses less. 



In minimum temperatures 5 inches above the surface on cranberry- 

 bogs considerable difference, according to the density of the vegetation, 

 is reported by Cox,'*^ from observations made in September, 1906. Table 

 61, which records his observations for the coldest nights, shows the magni- 

 tude of these variations attributable to the difference in the amount of 

 vegetation and the effect it has on soil temperature. Similar inequalities 

 may be expected in very weedy and dense strawberry beds. More frost 

 damage has been observed in weed-infested German vineyards than in 

 those kept clean. '^^ 



Table 61. — Minimum Temperatures with Thick and with Thin Vegetation 



(After Cox''') 

 (Degrees Fahrenheit) 



The effect of mulching, a common practice in strawberry growing, 

 should be mentioned at this point. As a winter protection the value 

 of a mulch is indubitable. In early spring a mulch tends to retard 

 blossoming, an effect which may or may not be desirable. Once the 

 plants are in blossom, however, a mulch may invite frost damage. 



Lazenby'O' reported observations to this effect: "To compare temperatures 

 over mulched and unmulched ground I took 16 observations with a self-registering 

 minimum thermometer daily between May 17 and June 1 of last year. The 

 average minimum over straw was 43.2°; over bare ground 46.4.° The greatest 

 difference was 7°. This year the average minimum over straw was 32.3°; over 

 bare ground 34° with a maximum difference of 3.5°." 



This effect is due probably to the exclusion of sunshine from the 

 soil during the day and to increased radiation at night. If the mulch 

 is used to cover the plants during frost, its effect is, of course, totally 

 different. 



Influence of Soil Moisture. — Observations on surface temperatures 

 in wet and in dry sanded bogs at Berlin, Wis., in 1906, indicated a 

 consistent and at times considerable, difference. Table 62, compiled 



