I have tried both, but where I live now I dont have a choice. My soilis so shallow I have to trench.

I tried an experiment about 10 years ago when I lived in an area with deep soil. I planted one large healthy plant with post hole diggers about18- 24 inches deep. Then in the fall I dug a trench along side of that plant so I could examine the roots. The root ball that was down deep in the soil had not developed much more than when I had planted it. Most of the good healthy roots had sprouted out from the stem and were no more than 6" deep (they were in the warm soil). The plants I planted by using the trench method had a much more healthy root system with roots all along the stem. They also out grew and out produced the other plant.
I have never planted deep after that. I figure that if the plant is watered properly the roots will grow as deep as they need to be. I have had good luck with the trench system, but your conditions may be different.

Larry

Raised beds are indeed a factor.

When planting tomato plants at grade level, I have grown tomatoes both ways and in average, well-drained soil in an average year (rainy spring, fairly dry summer with sporadic rain, and then a fairly rainy autumn), I don't think one method is any better than the other. The plants grow fine and produce fine either way. That's the short, simple answer.

However, gardening in our climate often is not that simple. So, here's the longer and more complicated answer.

The differences between deeper planting in holes or more shallow planting in trenches show up if you have difficult soil or a prolonged period of very dry or very wet weather.

WITH SOIL THAT DRAINS WELL:

In a dry year, if you have soil that drains quickly, you'd want to plant deeply as long as you make sure to water deeply so the water reaches the root zone.

If water rationing is common in your area in dry years, you might want to plant in a trench so that the soil can reach the roots in the brief periods of rationing when water is allowed. However, you also need mulch on top of the soil to keep the more shallow roots cool and the soil moist.

WITH SOIL THAT DRAINS POORLY AND STAYS TOO WET IN WET YEARS:

In tight, dense, compacted soils, particularly those with a high clay content....

In wet years, I'd plant high in a trench in the raised beds so the roots are less likely to become waterlogged.

In dry years, I'd plant more deeply but only if I knew I could water deeply as well. You cannot let dense clay type soils get too dry in dry years or water ponds on top of the ground and either rolls off or evaporates before being absorbed. So, while planting deeply helps, you need to mulch deeply as well. Any rain water that falls will percolate down through the mulch to the soil instead of running off bare soil quickly.

However, to be honest, I wouldn't plant at grade level in a dry year if I had dense, slow-draining soil. I would plant at grade level in a dry year in well-amended soil that drains pretty well.

Now, with raised beds, which is how I generally grow tomatoes:

Since my main garden's soil is mostly clay, even my raised beds have a high percentage of clay in them, but it is very well amended so it both holds water well and drains well. After 15 years of amending, it is a loamy clay. The clay ground underneath the beds (and this is a key point) also is amended so it drains better than the unamended clay outside the garden.

In my well-drained clay-based raised beds, I vary the planting method with the year....handling a very dry year one way and a very wet one a different way.

In a very dry year, I plant deeply and I also water slow and deep with either drip irrigation lines or soaker hoses. The raised beds also are heavily mulched to keep the soil cool and moist, and even the pathways are very heavily mulched in between raised beds in order to insulate the sides of the beds from sunlight and heat. If my raised beds are 8" above grade level, I often will put 8" of mulch in the pathways so that really, just by looking, you cannot tell the beds are raised.

In a very wet year (2004, 2007 and April-June of 2009), I plant in shallow trenches in the raised beds in order to keep the roots high enough that the heavy moisture and perpetually wet soil won't negatively impact the plants. I mulch very lightly---just enough to slow down the weeds. I'd rather have the soil exposed to sunlight and wind in order to help dry it out. Sometimes even that isn't enough in our clay, but it is about the best I can do.

In 2007, neither deep planting nor trenches worked. We had flooding rains in May-June, and I put as many peppers and tomatoes as possible in large containers because even the raised beds stayed wet too long.

In 2009, after getting 12.89" of rain in 24 hours late in April, we had massive issues with wet soil for about 3 months. Most of the tomato plants that had been planted deeply in raised beds (we had severe drought and horrific wildfires prior to this drought-breaking flood) set there with stalled growth for at least 6 weeks, looked awful and seemed near death for a long time. Since the property adjoining ours to the south sits higher than our property, water from that property contained to drain down onto ours for weeks and weeks, but it was draining underground where you couldn't see it. At the lower end of my garden, I could dig down 8 or 10" and hit standing water. I took the back-up tomato plants I had left over and planted them in the one raised bed I hadn't filled with plants before the monsoon began. Even the raised bed was horrifically waterlogged, so I made mounds of soil on top of the raised bed....picture a raised bed with nice tall gopher mounds on it. Then, I planted a back-up tomato plant in the middle of each of those mounds. That's the only time I've ever felt like I had to plant even higher than the raised beds.

So, as I hope all this explanation shows.....you have to choose the best planting method for your plants based on how well your soil does drain or doesn't drain as well as on the type of moisture you expect. It is generally a guessing game, but after a decade or so of gardening in the same place, I always can trust my instincts to tell me what to do.

One more comment about raised beds. The reason a person should build raised beds is to give them a planting area with great drainage. Raised beds work best for people who have dense, tight, compacted soil....usually with a high clay content, although you'd be surprised at how tightly sand and silt can compact over a long period of time too. To raise the bed, you start with the native soil and amend it. If you have really dense clay, you need to break up the grade level soil first and amend it, then build the raised bed on top of that. By using amended native soil in your raised bed, it is similar to the soil beneath it at grade level and I think that is important in terms of how the water percolates down through the raised bed and into the soil beneath it. (Also it is important in how the roots grow downward as well.....)

If your raised beds are very sandy soil or a very light-textured soil-less mix that drains very quickly but it is sitting directly on top of dense clay.....well, that's asking for trouble. You'll have issues with your raised bed drying out too quickly in hot weather while the soil beneath it can hold too much water most of the timel If I had gone to that much trouble to import soil for a raised bed and used 100% imported soil that was not mixed with native soil, I'd plant in trenches that kept the roots mostly in that imported soil. But, I wouldn't have built a bed that way anyhow.

To me, if you build a raised bed because you need it for improved drainage, then it makes sense to plant high in trenches. Otherwise, if you are planting in deep holes, you are basically digging down in the raised bed to the soil beneath it in order to put the majority of your plants' roots in the original soil beneath the raised bed. Why, then, have a raised bed at all?

However, there's so many extenuating circumstances here.....

In a hotter, dryer year you might need to plant deeply in your raised bed anyway in order to keep the roots down deep in cooler, moister soil.

And that is why there is no real answer to which way is better.....because there's so many different combinations of variables in terms of temperatures, moisture, the ability of soil to hold and release moisture, etc.

None of us has exactly the same soil or the exact same weather, so it is important for each person to experiment with different gardening techniques to see what works best for them in their soil and in their weather.

Hope this helps,

Dawn

Does this apply to containers as well? I usually plant deep because I don't have enuff surface area to trench plant.

Susan

Susan, I think containers are a completely different story. I dont think they are deep enough to cause a problem, plus they will be warm from top to bottom ( maybe too warn). I think that in containers you will find a very large amount of the roots at the bottom anyway,seeking water and cooler soil.