[MyAchingBack]

I am putting a couple of raised beds in my allotment, the dimensions being 6’ x 4’ x 9’’ x 1.5’’. Someone has suggested that I put a breathable membrane at the bottom of the beds to suppress any weeds. I’m guessing that as the beds will be 9’’ high no veg will grow deep enough to make contact with the membrane. Is this advisable?

[BabbyAnn]

It depends on the weeds - 9 inch of soil/compost on top will be suppressant enough for a lot of annual weeds but perennials such as couch grass bindweed and marestail will simply grow up to the surface so I see the benefit of a barrier providing the boundary of the raised bed is on top of the material.

Personally, I'd dig the weeds out first so that potentially the depth would be greater than 9 inch to allow for roots to take advantage of nutrients and water in the soil, especially during a dry spell.  Also, some root veg like parsnips can go quite deep.

[mumofstig]

You'd be surprised how deep the roots of some veg grow, although some will cope with 9in of soil by sending roots out sideways.
Root crops would struggle with shallow beds.
It's always better to start on weed-free soil and not use a membrane, but if you are only going to use them for stuff like salad or strawberries which are shallow rooting, then 9in should be ok

[MyAchingBack]

Thanks BabbyAnn. I didn’t really like the idea of using a membrane when the quality of the soil in the allotment is quite good. Incidentally, how deep do potatoes grow? 

[MyAchingBack]

Hi mumofstig. Will a 9in raised bed without a membrane be sufficient if being used on weed-free soil, or should I make the raised bed higher? It’s my first year with an allotment and I’m so worried about making a mistake 

[mumofstig]

If your soil is good, why build raised beds, or is it for the aching back 

Most people that use them make them about 6 - 9in high, but unless you have bad soil, or a bad back, I don't see the benefits. Spend your money on the soil, rather than on wood is my opinion 

You can still have narrow beds, that you don't walk on but use the soil that's already there.

Potatoes are planted about 4-6 inches below the surface and then mounds made over the top.
so I think if you used a membrane it would be in the way.

[Salmo]

What weeds do you have that you are so worried about?

Most of us wish for a good deep topsoil so that roots can penetrate and get moisture and nutrients. You seem to want to limit your plants.

To my mind if you have a membrane in place you can give up growing potatoes, brassicas, root crops, beans, sweet corn.

[compostqueen]

In the bottom of a deep raised bed the membrane wouldn't be a problem

I'd use thick sheets of cardboard though as it's good enough for this job and membrane could be used more wisely elsewhere on the plot.  I laid mine in the bottom of the raised bed over the weeds and they never surfaced.  I used fresh thick cardboard from the electric showroom, who were happy to get rid of it

Raised beds are very good for growing lots of things, especially those with deep roots.  If you have a bad back they are excellent. 

[Wavertree Red]

I used a role of Wilkinsons weed suppressant, 25' x 5' for £4.99, for my scaffold board raised beds. Had no problems with any crops but I was restricted to growing smaller sized carrots.

I blend a mixture of compost and top soil from an unused (and unusable) part of the plot to fill the beds. I've found I'm doing a lot less weeding than before.

[savbo]

I am putting a couple of raised beds in my allotment, the dimensions being 6’ x 4’ x 9’’ x 1.5’’.

wow, a 4-dimensional raised bed, I'll go and get The Doctor....

seriously, I wouldn't use a membrane on a 9" bed. Spuds need more than that, and while you can plant shallow and earth up that's not as easy on raised beds as on deep/lazy beds. My parsnips are nearly 2' long so they'd be out.

Suppress the weeds when you're constructing the bed with cardboard as CQ advises, after digging over everything you can. If you've got nasties like Jap Knotweed or Horsetail they'd get through membrane eventually anyway

[Trillium]

I toyed with the idea of membrane in the bottom of my wood-sided raised beds but didn't do it in the end. Instead, I forked out the bigger weeds then laid heavy cardboard a few layers thick all across the bottom inside the bed. Then I heaped soil mix inside. The bed boards themselves sat on 10" wide strips of membrane and where it extended outside the bed, I topped that with pea gravel to hold it down. So far this has worked very well to keep out most weeds outside the bed, and nothing has gotten through inside at all.

Due to the moist soil inside at the bottom, carrots and such can easily penetrate the now rotted cardboard and into the original soil below (not that there's much there for them).

My own raised beds are 18" high with about 14" of soil. This allows me greater freedom to rotate crops or even grow shrubs like blueberries which have 2 4x8' beds of their own. I find the wood sides contain the soil better and I can also sit and rest on the edges. The greater height allows me easier weeding, what little I get that blows in on the wind.

[Ice]

Absolutely agree with what has already been said, if you have good soil and a pain free back then raised beds are not necessary.  My beds are about 7"-8" deep on a bed of membrane.  This season they will be considerably deeper as I fill them with whatever I can make or afford to buy.

I had great potatoes last year despite the depth and It was due to good soil preparation, ie lots of well rotted manure, earthing up and regular watering.  Root veg are a bit of a challenge but my chantenay carrots were just the best.  I even grew a giant pumpkin in one of the beds the year before last.

[MyAchingBack]

Thanks everyone. Raised beds and no membrane for me.