Make this garden bed into a place for my fig trees.  The rains in the spring are so much it drowns out my vegetable garden each year.  So, this soil was placed directly over lawn grass and is about 18" deep.  Do you think the roots of the trees will sink into the natural ground and take root there? The bed is 10 ' by 5' I want to plant my trees inside, what do you think of this idea?

The figs will happily send their roots into the natural ground.  NP!

Suzi

I have wet winter soil problem, mature grape plant dead, mature Japanese maple dead, mature apple tree dead.

I planted 4 4FT tall test fig trees in ground near end of last year, I checked them at spring time, 2 of them was root rotten due to wet soil, I build them 10 " raised bed but due to roots damaged plus new soils are not break it yet mistake( soils was still composting), They do not grow well, no fig for this year, very disappointed.

I planted a disappointed fig tree ( wrong variety, 6 years = 9 edible figs) in 10" tall raised bed, it grows fine, I think that your garden bed should be fine for fig tree, but once you plant it there, Its roots will take over and you can plant flowers but not tomatoes.

If you wish yo have a large tree, dig up the soil, plant your tree and include some quality soil mixed up with the surrounding soil at least Two to three feet radius sloping down gently away from the base of the trunk. Your mound should be no more than a few inches and sloping down gently away from the base of tree. This would let the tree get acclimated to the surround soil asap.
My next door neighbor was a landscaper and told me that in the Austin area unless you do it this way, the probability of losing the tree two to three years down the road is high.
I noticed that your yard is gently sloping. If that's the case it should provide you with some drainage.
I would do it something like this.

I know it will work.

Early last year I planted a JH Adriatic in a raised bed that had previously been used for tomatoes, it grew very well. Easily 60 feet or more of wood in one season, one branch alone grew 10 feet.  I'm growing it as a step-over and it should fill the entire space by next year.  My raised bed was mainly composted barn waste and very rich, well draining 14'x3'. 

My soil is different than Austin's, but I'm thinking yours will do well.  Those roots do not stay inside the bed, and will travel.  Also, as others have said, you will not be able to plant anything else in there for very long.
One fast growing tree will fill that entire bed quickly especially if the soil is fertile.

If you want to continue to use bed for other things, doing what Sas recommended is always a good idea.

I would keep your beautiful raised bed for your garden and just make a small raised mound for your figs, unless your yard is VERY wet - as in standing water for a week at a time wet. A 8-10 inch tall mount, 3 ft in diameter will be plenty to keep your fig up in the dry. 

Hi,
You could plant 2 or 3 trees in there, or just one in the center. The choice is yours. I would go 3, 1 in the middle next to us in the pic, and the two others in the corners far from us.
If drainage is still a concern for you, you could plant in buckets with bottom removed (raised bed in the raised bed).
Half of the bucket over the dirt level, and half buried in the dirt. That will add additional volume and drainage.
It will help you too when you want to till the dirt as the roots of the fig trees will escape under the bucket.
The roots will then reach the lawn and if they should rot, the tree would be left with enough roots to rebound.
I have some trees planted like that in gallon pots or 80liters/20Gallons trashcans with bottom removed - did I mention that I hate rodents ...- and they are doing fine.

In fact, the ones in gallon pots with bottom removed are doing better for one easy reason (once you've seen it) : They can reach the water from the surrounding dirt easily, when growing out of the pots.
With the trashcans, the roots don't seem to escape the container, and in the heat of the summer they suffer from lack of water - when I can't tend to them .

Go ahead plant 3 and send us a pic :) .

Now that is EXTREMELY impressive!