Bulb frame construction - advice welcome

[Tomte]

I am currently seeking to build (or let build) a new bulb frame. One of the reasons is that I recently relocated from Munich to the surrounding countryside and am now living at 700m above sea level, which presents a challenge for growing a number of bulbs. I already lost several species to frost damage this winter/spring.
Having not a huge garden, I have to make the most of the available space. Thus, I intend to construct the frame as part of a raised bed in front of a south-facing wall. In that regard, I have some important questions:

• What power/m² would be appropriate for a heating mat in order to ensure the ground does not freeze solid for prolonged periods in winter and spring? Being cost-conscious, a power as low as possible would be preferable. This question needs an answer soon, as I intend to make use of this in my previous bulb frame, which needs to be set up within the next 3 weeks..
• Can anyone recommend a supplier or a firm constructing such frames/alpine houses in the greater Munich area or at least in southern Germany?
• If not, (or perhaps even then, because it's always a good idea to have an idea about how the design should look in the end before asking someone to do it) which design would be most suited to fit the circumstances (e.g., how best to manage front access, where and how to place vents and how to ensure proper shading)?

I will post pictures of the site soon and might also attach a sketch of how I imagine the design and would be grateful for lots of good advice 

Thanks, Tom

[Steve Garvie]

Hi Tom, In my climate I have to raise my bulb beds well off the ground ( due to heavy Summer and Winter rain). As a result I lose some of the thermal buffering that a bed at ground level would have. I therefore insulate my raised beds with 5cm thick polystyrene sheeting. I lose a bit of growing space as a result but I find this very effective at buffering severe temperature swings. I also use subsoil heating cables placed about 16-18cm deep in beds containing frost-sensitive bulbs. I space the coils of the heating cable about 5cm apart at the edges of the box and lay the cable from the outside inwards. With each subsequent run of cable I increase this gap as I work towards the centre. I use a thermostat to control the heating cable (https://www.ebay.de/itm/Inkbird-ITC308-Temperature-Controller-Termostato-Digital-Heater-Fan-Control-220V/222373505143?epid=2296084805&hash=item33c67e6877:g:ubQAAOSwbA9dJtPt). Include a reasonable amount of perlite in the bulb frame soil mix as it improves aeration but also has some insulative properties.


Waterproof box housing thermostat and connections)



A raised bed frame built from old railway sleepers, treated with preservative, lined with plastic “damp-proofing” membrane and polystyrene sheets (top of one sheet visible at far end of frame). This frame is used to grow lowland Chinese lily species such as bakerianaum and amoenum -these are not frosthardy so need protection from soil freezing but rise late in Spring after late frosts so this frame has no sides. My other bulb frames have glass sides.


(Lilium yunnanense)



(Lilium bakerianum var. delavayi)



(Lilium bakerianum var. rubrum)


(Central Asian Corydalis/Tulip bulb frame)

[Hans J]

Hello Tom ,

Here is a answer from a other collector ( also born in Bavaria )
I have a greenhouse from https://www.princess-glashausbau.at/ since 1993 ...and I'm very satisfied
From the same seller I have bought some year later a bulb frame which is also good
I live here in a very mild climate ...so I use no heating ( there are not so sensible bulbs like Sternbergia ,Acis ,Rhodophiala ,Scilla ,Narcissus
In winter time I have automatic window openers  ( in case it is too warm )in summer all bulbs inside are dry ( no water from April until Sepetember ...the windos are a little bit open ....and I cover it with a  Shading net
If you have more questions plase write me a PM ...it is easier to answer you in german language

Good luck
Hans

[ArnoldT]

Tom:

One of the things I did when building the greenhouse foundation wall helped a lot to keep the floor well insulated.

You will want the plunge material insulated from the soil on the outside of the bulb frame.

I used a poured slab of concrete with a radiant tubing system to heat the entire greenhouse.   It is supplied by a small furnace in the basement.  The greenhouse abuts one of the exterior wall of the greenhouse.

I had the contractor install a foam board with  aluminized surface facing the inside of the greenhouse.  One of the greatest heat loss area is through the foundation wall.  Since the temperature of the area just inside the wall is higher than the soil on the outside of the wall it is a high potential for heat loss.  Prior to the pour I installed a twelve inch deep sheet of the foam board on the inside of the foundation wall.

Essentially the floor is separated from the exterior wall by a 1 1/2 inch sheet of foam board. I can take a picture of send in if you desire.

[Tomte]

Dear Steve, Hans and Arnold,

Thank you for this valuable Information. One thing I‘m particularly curious about, as already mentioned, is how much heating power is required or might be at least agreeable. In particular, because heating cables require some sort of fixation (maybe I‘m also overly cautious/perfectionistic in that regard), I like the concept of a heating mat. However, most of them use a lot of energy (one model I find suitable from a quality perspective uses 2500 W at an area of 53x44 cm. I wonder if that‘s overkill and/or uneconomical, or if the actual power does not matter when used in conjunction with a thermostat.

Steve, what’s the input of you heating cable and what‘s the heated area?

[Steve Garvie]

Hi Tom, if the frame is well-insulated you don’t need a lot of power. The heating cables are easy to place in the soil and are held in place by the soil (or sand plunge). Remember their function is only to prevent penetrating frost from damaging the bulbs. The heating cables come in various sizes -6m (50watt), 10m(100watt) and 25m(320w). A 6m (50w) cable covers about 0.8 sq m of the frame. If you are growing plants with very frost-sensitive foliage then they are better grown in a greenhouse than in a cold frame.

[ArnoldT]

Tom:

Here's a link to what I've done in my radiant floor greenhouse.

You can extrapolate it for your cold frame.  Heat loss will occur through the side walls and down into the subsoil.

https://www.finehomebuilding.com/2014/07/09/insulating-a-slab-on-grade

[Tomte]

The advice I got here was amazing. In the absence of enough time (a law exam, the construction of a jungle gym with sandbox for my son and numerous other things around the new house got in the way) this year I put off the original plan to get a proper frame in place until next year. The Austrian firm sounds promising, although I have to confess I did not contact them yet. Maybe until then someone might have further ideas?

Anyway, for this season I decided to migrate my old frame (which I enhanced with styrofoam-insulated walls) to the nee location an sink it into the ground there. Thus at least the sand is already in place. I obtained an 8m (60W) heating cable with a thermostat from a small German supplier which is supposed to be accurate and robust. A picture of the cables laid out in the frame is attached. The sensor is placed below the central stabilising rod. As pointed out by Steve, perlite has good insulating properties, so the bottom part of the frame is filled with a 40/60 mix of perlite/sand.

In the attached pictures you may also see the site where the frame is now situated. As I said, the wall is south-facing, so I guess I have to figure out how/where to install proper shading soon..

I currently have no picture of the top part of the frame but I’ll take one soon once this year’s project is concluded.