Drainage for Raised Bed on heavy clay soil

Khaiba

Seedling
Messages
18
Reaction score
3
Location
Germany, Baden-Württemberg
USDA Zone
8a
Hi all,

I'm trying to build a raised garden bed (120x80x40 in cm, about 50x30x15 inches) for growing prebonsai and thickening trunks.

Having the raised bed sit on my clay-heavy top soil (open bottom), do I need to be wary of anything drainage-wise? (would the water pool around/beneath the bed?)
I heard people suggesting to put a drainage layer at the very bottom; however some others said there should be no layers so it can "match" the top soil of the garden.

As for soil, I have bought about 1/3 potting soil, 1/3 compost and plan to fill the rest with top soil (mixing them all together).
Is there anything else (preferably inexpensive), that I should add to the mix?


Additional notes:
1. I live in Europe, zone 8a
2. Going from a small home-made test I made, I would say the soil in our garden consists of about 50% clay, 25% sand and 25% silt
3. Plants I want to grow in the bed:
- JBP, JWP and Scots Pine
- Acer palmatum
- Chinese Juniper and Hinoki Cypress

Thanks for reading! I'd love to hear any type of advice, experiences or thoughts :)
 

Shibui

Masterpiece
Messages
2,718
Reaction score
5,184
Location
Yackandandah, Australia
USDA Zone
9?
It should not matter what soil is below an open bottom raised bed because excess water can get away. The clay soil below may even be helpful when some roots get down there as clay is usually quite rich in minerals and holds water well.
potting mix/compost/topsoil should be a good mix for a raised bed but probably not good in smaller pots so shake as much off the roots as practical when moving the trees into pots at the end of grow cycle.
Pines an juniper can be quite slow to grow, even in the ground. They take around 5 years here and may be longer in Germany but that obviously depends how large you want them to get. Acer palmatum should grow faster so allow space for digging those before the conifers are ready.
Conifers will require pruning and training while they grow to avoid strong, straight trunks.
 

Khaiba

Seedling
Messages
18
Reaction score
3
Location
Germany, Baden-Württemberg
USDA Zone
8a
Thanks for your reply Shibui!
Would it be safe to plant them at this point of the year into the raised bed? Without root pruning of course, just placing the trees along with the root ball into the bed?
 

coachspinks

Shohin
Messages
479
Reaction score
470
Location
Just south of Atlanta
USDA Zone
8a
Thanks for your reply Shibui!
Would it be safe to plant them at this point of the year into the raised bed? Without root pruning of course, just placing the trees along with the root ball into the bed?
Welcome! My mothers family is from Ansbach. I spent a good proton of my teen years just east of you.

You should be fine if you don't disturb the roots.
 

Shibui

Masterpiece
Messages
2,718
Reaction score
5,184
Location
Yackandandah, Australia
USDA Zone
9?
They will survive transplant now without touching the roots.
Big question is what are the long term ramifications of such a transplant?
Nebari is not such a big deal for juniper but they look better with good roots. Nebari is very important for good maple and pine.
Just planting from pots the roots are likely to be tangled. Small tangled roots will grow into larger tangled and fused roots while the trees thicken. What will those look like when you have nice thick trunks?
One year I did just that. Late planting of JBP straight from plastic pots into the grow bed. When I dug them a few years later every one had a fused root mass exactly the same shape as the original pot (reverse taper because of pot shape). Some were able to be salvaged after intense root work, some were scrapped.
I would strongly advise against just slipping intact root balls into a grow bed unless you are certain the roots are already good. Saving a few months is not worth wasting several years growing inferior trees. If the trees are not too root bound it is possible to split the root ball and spread it out without too much damage to roots. Trees can tolerate that much any time but it is a poor second to transplanting properly.
 

penumbra

Masterpiece
Messages
3,015
Reaction score
3,648
Location
Front Royal, VA
USDA Zone
6
About the original post. First, it is correct that it doesn't matter what the native soil is like as long as your raised bed isn't sitting in a hole.
Second, small raised beds like you are planning are very easy to build and set up but they do not hold much once things start to grow.
Third, 15 inches deep is excessive, half of that is fine.
I am experimenting now with small raised beds but have little to report at this time. I will follow up when I have learned more. My original bed that was installed and planted spring of 2019, is 8 feet long x 3 feet wide by 7-1/2 inches deep. It has landscape fabric on the bottom along with galvanized 1/4 inch hardware type cloth to keep rodents from entering from the bottom. It is filled with just non amended topsoil (not the crap in bags that they sell as topsoil). There is a mix of plants in it including Fuji Cherry, larch, amur maple, dwarf magnolia, umbrella pine and a few others. Everything has done very well and some things have exploded with growth (like the cherries) and need to be replanted to space them out next spring. This non amended topsoil is as close as I could get to planting in the ground.
I have currently 4 other raised beds constructed to experiment with mixes of substrate and planting methods that are 6 feet x 2 feet by 7-1/2 inches, requiring only two 8 foot x 8 inch boars. One has nothing but coconut choir. One is nothing but a peat based grow mix amended with pine bark fines. One is coconut choir, grower mix, Napa 8822 and pine fines. One is not filled yet but I would love to use a purely inorganic mix if it was not so expensive here. I also just built a bed like those described above that is 8 feet long that will be a seed bed which will probably use a mix of grower mix, coconut choir and perlite.
I am really enjoying the process and hope to learn a few things I can pass on.
 

Khaiba

Seedling
Messages
18
Reaction score
3
Location
Germany, Baden-Württemberg
USDA Zone
8a
Just planting from pots the roots are likely to be tangled. Small tangled roots will grow into larger tangled and fused roots while the trees thicken. What will those look like when you have nice thick trunks?
One year I did just that. Late planting of JBP straight from plastic pots into the grow bed. When I dug them a few years later every one had a fused root mass exactly the same shape as the original pot (reverse taper because of pot shape).
This is my main concern. However, I plan on root pruning them and repotting them on a tile next spring, so I hope the roots will not become too much of a mess until then. Having no real experience with field growing unfortunately, my predictions could be dead wrong. The plants that I plan to plant are between 2-4 years old, not quite sure how strongly they will develop roots in the first 6 months or so.
Clearing my balcony from the army of little plants and pots would be a huge relief though.

I have currently 4 other raised beds constructed to experiment with mixes of substrate and planting methods that are 6 feet x 2 feet by 7-1/2 inches, requiring only two 8 foot x 8 inch boars. One has nothing but coconut choir. One is nothing but a peat based grow mix amended with pine bark fines. One is coconut choir, grower mix, Napa 8822 and pine fines. One is not filled yet but I would love to use a purely inorganic mix if it was not so expensive here. I also just built a bed like those described above that is 8 feet long that will be a seed bed which will probably use a mix of grower mix, coconut choir and perlite.
I am really enjoying the process and hope to learn a few things I can pass on.
Very interesting! I also plan on growing some Fuji Cherries. If you learn anything from those experiments, let me know! Inorganic material is indeed very hard to come by cheaply.
How much space did you leave between each tree? Also, does your raised bed have some kind of shade protection? The Japanese Maples that I have are very sensitive to just the smallest exposure of direct sunlight, so I was thinking of placing a shade cloth above those.
 

penumbra

Masterpiece
Messages
3,015
Reaction score
3,648
Location
Front Royal, VA
USDA Zone
6
This is my main concern. However, I plan on root pruning them and repotting them on a tile next spring, so I hope the roots will not become too much of a mess until then. Having no real experience with field growing unfortunately, my predictions could be dead wrong. The plants that I plan to plant are between 2-4 years old, not quite sure how strongly they will develop roots in the first 6 months or so.
Clearing my balcony from the army of little plants and pots would be a huge relief though.


Very interesting! I also plan on growing some Fuji Cherries. If you learn anything from those experiments, let me know! Inorganic material is indeed very hard to come by cheaply.
How much space did you leave between each tree? Also, does your raised bed have some kind of shade protection? The Japanese Maples that I have are very sensitive to just the smallest exposure of direct sunlight, so I was thinking of placing a shade cloth above those.
They were only about 6 inches tall and I planted them about a foot apart. Now they are completely grown together on top and it looks like one plant when you walk up to them. My grow bed gets only about 5 hours of sun midday. No problem with burning here.
 

Shibui

Masterpiece
Messages
2,718
Reaction score
5,184
Location
Yackandandah, Australia
USDA Zone
9?
Clearing the balcony is the only reason I can see for planting now. You may get some growth on the maples but conifers are typically very slow to respond in the ground so unlikely to get much growth above or below the ground in half a season. Dig and root prune in spring will be OK I think. I do not use tiles. Proper root pruning is far more effective in my experience.
Allow room for growth, especially for conifers. Shaded low branches get weak and die but those are the most valuable for bonsai. I plant 40-60cm apart in rows 1m apart for ease of access and maintenance and to allow room to dig when the time comes. Spacing will be related to the size you want to grow to. Further apart for more growth and larger trunks. Closer if you are aiming for smaller trees. 80cm wide does not give much space for 2 rows unless the trees are relatively small. Possibly plant 20cm from each edge and around 40cm apart? Staggered rows could work for larger trees. Now that I analyse your sizes I do not think you are going to get many of the army off the balcony. 120x80 is not a lot of space so I hope you are not planning on large size trees. Closer spacing will still give some growth but may lead to problems when you want to dig the trees.
 

penumbra

Masterpiece
Messages
3,015
Reaction score
3,648
Location
Front Royal, VA
USDA Zone
6
Closer spacing will still give some growth but may lead to problems when you want to dig the trees.
This is whay I just filled one of my beds with small trees in inground grow bags. I had experience with them many years ago and not only did they simplify things but I loved the results.
 

Similar threads

Top Bottom