Raised Bed over Old Driveway Dispute

This weekend my DH and I cut and lifted a 4 x 12 slab of concrete from the side of our driveway. It's going to be a new raised bed for vegetables. Underneath the slab is a 3" base of sand and gravel. Under that is the subsoil. I think that we need to remove all of the old base, so that the new topsoil I'm getting will be in direct contact with the subsoil. He thinks that we can just leave the base there and it will create a drainage layer. This, to my mind, would be the equivalent of putting gravel for drainage in the bottom of a plant pot - a no-no in todays thinking. He intends building the raised bed to only 1ft on top of the driveway edge which will leave 4-6" below bring the total depth of the raised bed to 18"max. If we remove all of the base it will be closer to 20" deep. Is there an optimum depth for raised beds? If we left the base in place should we build the bed higher to 2 or 3 feet? What effect will leaving the base have on my plants?

Please help!!!

This message was edited Jan 20, 2008 7:12 PM

Very interesting question! I don't know the answer! Seriously though, drainage is interesting and can be confusing. I think that drainage layers in the ground are different than in a pot but I'm not sure how, just what I have read.

I do have an idea and that would be to do an experiment. You would remove all of the base from half of the garden and compare how things grow. I cannot think of a better way to know what to do besides to do it and see what happens. This sounds like a good compromise as well.

If the area with the base gone does better, then next year you could dig the base out from the half that still has it, too or if it is the other way around you could put the base back if you still had it. Or you could leave it half and half.

It may be that some plants would do better on one side than the other.

Let us know how things turn out.

David

I'm going to have side with your hubby. The sand and gravel won't hurt a bit and will probably help make your beds more productive in the long run. He does need to build that bed another ft deep at least. I would recommend that in any case, whether you moved the base or not. Our raised bed are usually at least 2 ft deep and mine are side less, we just shovel the soil on top and pat it down. It gives your plants a chance to build a strong root system, giving you healthier plants in the long run.

I'm siding with remove the sand and gravel - and loosen up the subsoil. My thinking is about contamination with the concrete that was on top of it - working with vegetable beds I would go for a clean surface. Unless you are on really boggy soil, drainage is rarely an issue with vegetables, and if I am right, even northern california isn't all that wet these days. I would loosen up the subsoil because it has been compacted to put the drive over it. I think raising the beds to 2 foot raises issues about access and appearance - I would go down first, rotavate the subsoil just to remind it is is soil and not hard standing, and then add a good 18" of top soil/compost/manure. Then do it again next year. But then I like a little effort with my carrots - well done to get rid of the concrete and plant up some veggies!!

I'm beginning to think that there are going to be an equal number of arguments on both sides :-)

We have to build the raised beds with sides because the ground is slightly sloping, so sideless is out. And the way the rains have been here in Northern California in the last 2 years, it's either dry or flooding. Much of the lovely organic matter gets washed out of my low lying garden beds and just floats down the drains and I have to start all over again. Drainage is quite good though and we rarely end up with a boggy mess.

I've shown DH all the responses and he likes Laurie1's the best. He doesn't want the expense and work of building the garden to 2ft (cementing the posts, extra timber and topsoil) and since I have convinced him to do this job for me I have to go along with that. Kind of....

The topsoil that is sold around here is sandy-loam. Is that suitable for filling the bed? What mix of compost/topsoil/manure would be right? Just which vegetable requires the deepest root area? Tomatoes, carrots, ... ????

Carrots, celery, beetroot, potato love a sandy loam - pumpkins, beans, tomato, brassicas like it quite retentive - so it really depends on what you are growing.

Check what you are getting, if it is really open, you can always add manure to put more organic matter back in. Don't be afraid to ask questions of your supplier - they want to give you what you want. Does your supplier also carry other types of materials: manure, compost, mulches? If they do, they may be able to make a blend for you - worth checking. (If you are getting sandy loam, glad you are removing the sand/gravel!) - If the supplier can't do the blending, you can used a well washed out cement mixer to make your own blend of well rotted manure/compost and ts. The cement mixer makes pretty large batches, and its really worth doing. I would say retentive is preferable, you'll still get really good growth for the sandy loam veggies with new soil.

Enjoy yourself, and don't forget to give DH an extra kiss - good man!

I wish my 2007 garden had a layer of gravel over a foot deep. In 2006 could not get enough water, but in 2007 lost most of my crop to slow draining water. Potatoes rotted very early with the first wave and almost everything else went in the next couple of rains. Large acreage crops have been spending fortunes putting in drainage tiles quite alot the last few years and you already have something similar. I would leave it. Just my 2-cents and my wasted 1/4-acre garden on a clay base. Take care,
Mike

There are two conditions that could affect drainage. The first is the level of compaction in the base layer. If, for instance, the "gravel" is crushed limestone & presents a barrier through which water will not percolate, it will present some difficulty - especially so, if the surrounding soil impedes lateral water migration. The second issue is the size if the particles in the base layer. If the particle size in the base layer is large, water could "perch" in the soil above & not drain. Particulate size in the drainage layer should be no larger than 2.1 times the size of the particles in the soil above. This would only apply if there is a substantial layer of gravel on top of the sand. If sand & gravel are mixed together, consider it a nonissue.

Al

LOL Laurie - I'll do just that.
I checked the suppliers website and was pleased to discover that they do indeed sell manure plus assorted other composts and mulches. And thanks for the tip about using a cement mixer to blend it. Very handy.

Mike, sorry to hear about your garden. Was 2007 particularly wet in your area? I hope you have a better growing season this year.

The base layer is pretty well mixed up and none of it is limestone - just aggregate & sand. Yesterday I used a lot of water washing the cement dust off the driveway and created a little pond in the new bed plus we had heavy rain today and it's all drained away nicely. Of course the lawn that borders this spot is quite soaked but thats fine by me - it's not Cricket season yet ;-)

wgnkiwi...There is a product out there call an Earth Box...it only has 8" of soil in it with 3" of water below that. Sounds to me 12" of soil would work very well

Love Compost Tea

Sounds like you are on your way - good growing.

I have 2 Earthboxes !!! Thanks for reminding me, rentman. Gosh, they make getting the watering so easy, don't they? I've never brewed Compost Tea but regularly make Alfalfa Tea during the warmer months. The plants just love it.

wgnkiwi...I bought an EB when I lived in FL and made 3 more, thay are very handy for the padio growing. Something happen to them on the move from FL to KY a year ago. I bought a new one last week at an auction for $20 , wheels, nutrient and all .
COMPOST TEA there is a lot of info about it on the web , but I just fill a 1 gal bag with compost , put it in 5 gal of water , let it soak for 2 days or so and use it as I would water , and the plants know what it is and love it. It's a wounder that you can plant a seed in dark dirt , feed it dark brown water and get a Red or Blue flower..............just love it.
This is my GH this AM

We had record rains last year ending a 3 year drought. We had 8.5" in one day in my neighbors rain gage, although the news said 7.5". Not planning on doing a garden this year. Going to try to concentrate more time on my fruit trees.

Sounds like you are moving right along. Take care,
Mike

Aww, cute greenhouse. I see photos of folks gardens covered in snow and think that maybe it wouldnt be so bad living in a colder zone, but then I go outside and it's 50F (like today) and I realise I'm too soft for such cold weather. Brrrr
Thank you for the compost tea info.

8.5"!!! My back yard and garage would be under water. Good luck with your fruit trees Mike.

I just stumbled on this one. My concern with leaving the gravel is the fact that roots and worms and bacteria and .... don't like to cross strata that is vastly different. I have always mixed the sub soil with my additions in my raised beds. I think any raised bed drains just fine, after all they are higher than any of the surrounding area. So who needs the drainage layer. Make that DH kick in and do it right. LOL

I would set that 4X12 up on blocks, buy the book "Gardening on hard surfaces" and create a Zone 9 rock garden having complete control from top to bottom with hardscape and perfect soil for the whole thing and seating all around for close up viewing of all your little treasures. Oh, yeah, and what Sofer said. hehehehe
Carol

well as most vegetables are annual, i don't think the roots will get that deep anyway. if they were perennials, it would probably just help in aerating the soil, so i wouldn't go to alot of trouble.

Well depends on which veggies, if you're going to grow things like carrots, beets, parsnips, you need good drainage and fairly sandy soil.

I wondered whether worms would make their way up through the gravel, or whether I would have to add them once the garden is completed.

I'm planning on growing a huge range of vegetables from carrots to potatoes to leeks to tomatoes in this spot. With some careful planning (& a sprinkling of fairy dust) I hope to grow just about everything we eat almost all year round, while using the earthboxes and my summer vege garden to hold the extra tomatoes I need for canning.

We got almost all of the gravel scraped out and bagged into sandbags before the rain came back. Just waiting for it to clear up a bit more before we get the rest out and start tilling the subsoil. Can you imagine how difficult it is to try and dispose of that stuff in an urban area? DH thought he might advertise it on Craigslist as "free to good home". LOL

Oh yes... I have turned out to be just like my father. My outdoor furniture is all ready in place for close up viewing of aforementioned treasures. What better way to end the day?

wgnkiwi, they were trying to find your exploding egg thread over in the rose forum. lol!

Wgn - can't think of a better end to the day - except possibly the addition of an adult beverage of choice.

Do they need a laugh in the Rose forum Len? I still get the shivers when I think about it.

Laurie - Got it in One !! When we're all finished you're all invited to come pull up a deck chair, grab a glass and sit for a while.

This message was edited Jan 24, 2008 8:09 AM

Nice.

wgnkiwi, sorry to be so late to your thread.
DH and our fellow community gardeners have hauled truck loads or broken concrete and gravel from homes that were remodeling their patios. The concrete was made into stacked drywalls (walls with no mortar. DH is a skilled stone mason) for garden beds and the gravel found a home on our garden paths.

Please do post the free gravel and other materials on Craigs List and Free Cycle. There are a lot of folks who can use it. It is not difficult to dispose of in an urban area if you let folks know it's available. It's been first come, first serve and materials can move quickly. So nice of you to bag the stuff. We've always had to shovel it from ground to wheelbarrow to truck before running off with it!

If you haven't already purchased your topsoil, to a moist soil squeeze test on the soil you are thinking of buying. We've purchased topsoil from a few garden/landscaper suppliers here. The quality varies significantly. Some of the topsoil is our local adobe clay that has been scraped off of housing tract sites in the greater area. Some mix it with compost, some don't. See if you can moisten a little of the soil you intend to use and then give it a squeeze. Does it stay in a tight clump, a loose clump or just fall apart? That will tell you how much you need to amend it.

Hey, g_m, your're not late at all. Progress on the raised bed has stalled because it has temporarily become a wading pool with all the rain we've just had. At 10pm last night it was 8 inches deep, at 5am this morning it was down to about 5" and by 10am it was all gone. That subsoil drains just fine :-p

Clever DH used the cut squares of concrete to create a path down the side of the garage (it was previously just builders muck) and a platform for our BBQ. I was sooooo proud of him for recycling it all - he's come a long way in a short time. LOL He has decided to keep the bags of aggregate for when he does some paving at the front of the house, but it's good to know that it could be "freecycled" if needed.

Thank you very much for the info on purchasing the soil. I'm hoping to get it next weekend and will definitely check it thoroughly. I would not have thought to do that if you hadn't posted.

Soils that do not drain properly are not a choice for varied plant success. Where I grew up we "tiled" the poorly drained areas and it provided a wide choice of plant success. I would look to drain the area with tile before I placed raised (submerged) beds in that area. That could be accomplished by diverting source of run-off or draining more effectively the area submerged.