With Ray’s recent photos of the peach, crabapple, and hydrangea planted too deeply, a discussion of tomato planting depth arose in the comments. I’ve seen the prolific adventitious roots start to form near the base of tomato plants, and I plant tomatoes to the cotyledon or deeper, but tomato planting depth not an area I have extensive research experience in. So I did a little literature search.
It seems that the practice of planting tomatoes is more than just friendly garden folklore. There’s some evidence that it works. Or, at least it works sometimes. Early in the season. And maybe not everywhere. Oh these darn scientists, with their wishy-washy caveats.
There just isn’t a whole lot of peer-reviewed research on the subject. One of the recent papers (from 1996) cites a “Crockett’s Victory Garden” (circa 1977) as a source of information for Northern gardeners about the benefit of deeply-planted tomatoes. Jim Crockett was no horticultural slouch, but just because successful gardeners do something doesn’t mean it’s sound practice (but like I said, I do it too…).
Southern tomato growers, on the other hand, have some peer-reviewed evidence to go on. It seems that in some years and some locations in Florida, tomatoes planted to the first true leaf were able to be harvested earlier (Vavrina 1996) than if they were planted to the top of the root ball. The overall trend was for a bit more and a bit larger fruit per plant. The paper is pretty sparse on detail (statistics and design), but that’s what they conclude. Similarly, a study of fall-planted tomatoes in Louisiana (Hanna, 1997) showed a benefit of planting to the first true leaf instead of to the top of the root ball in two years at one location. In that study, treatments that lowered the root zone temperature tended to increase yield. If you can get an extra 3 pounds of medium or larger tomatoes from a 50 square foot plot in some years, what’s the harm? Deep planting certainly helps prevent lodging in tender young plants, so there’s a clear benefit there if the supports aren’t adequate.
Most of the work suggests that, like mulch, deep planting helps to moderate root temperature, and fluctuating or high root temperatures are stressful to the plant. I can get behind that I suppose, but it’s hot in Florida and Louisiana in the summer. What about stuff planted in Minnesota, where I live? We use plastic up here to get warmer soil temperatures at planting, not cooler temperatures! Well, I didn’t find any work on that. But interestingly, some of the yield benefit seen in planting peppers to the cotyledon or true leaf in Florida (equivalent to 5 extra pounds per 50 square feet, in 3 of 4 years, with commercial spacing and fertilizer rates; Vavrina, 1994) don’t show up in Massachusetts (Mangan, 2000). But deep planting did help prevent lodging, which allowed for faster crop maturity in both places.
So the verdict on deep planting tomatoes? It doesn’t hurt, it helps sometimes, and it helps prevent lodging, so why not? One caveat: don’t plant grafted tomatoes deep. The scion will make roots, negating some of the benefit of the rootstock.
(And an addition from Linda: here’s a diagram from TAMU demonstrating planting depth for tomatoes; this link will take you to the article itself.)