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I needed some bottom heat for some smaller seedlings as well as for a few other trays of plants. I have been looking into building a larger propagation area but I also plan to move soon so I did not want to invest a lot into this system.
I first bought a few Heating cables. These are long cables around 150 to 200 feet long and 500 watts. These are called deicing cables used to keep ice off of roofs and out of gutters. The price was good for the size of the cables. I also bought some plastic mesh to keep the cables from touching and so that it would be easy to later roll up and move the mat.
Here is a photo of the heat cable weaved into the plastic mesh.
The cable stays around 90f and I placed the wires around 4 to 5 inches apart. I then prepared my greenhouse table by adding foam insulation down on the bench and then unrolling the heat mat on top. This is to keep the heat up on the plant roots. I also added a capillary mat to keep the roots moist and humidity high.
Here is a pic of the finished table.
and another pic of the the mat looking under to see how it's laid out. If their are sections that stay to hot dry out to fast I have been adding small cell trays to raise the plants up off the table 1/2 an inch or so so that the heat can be evenly distributed.
One might question whether the circumstances really require a heat mat or not. If temperatures approximate normal room temperature (20 deg C, 70 deg F), then scientific study would say not.
From Dr. N. Deno's Second Supplement to Seed Germination Theory and Practice, pg.3:
"Bottom heat and temperatures above 70F have often been recommended for germination, particularly of the common garden vegetables and annuals. Henry M. Cathey has already studied this question extensively, and his results were published in... 1969. He studied germination at five degree intervals from 50-85 F. Germination was usually optimum around 70, and temperatures down to 50, or up to 85 often result in markedly lower germination. I have studied germination at 40 and 70 for most of the species studied by Cathey and confirm that germination at 70 is satisfactory... Both Cathey's results and my own indicate that temperatures above 70 are neither necessary or desirable for germination... Why have temperatures above 70 been recommended so often? Many of the recommendations in the literature are inferential such as recommendations to use heating cables or place the seed flats on hot pads or other warm surfaces such as the top of a refrigerator. There are possibly two reasons for these traditional concepts. Many greenhouses in spring may be at temperatures significantly below 70 and particularly the soil temperatures are below 70. Bottom heat could be helpful. Secondly, a century ago temperatures inside houses were colder than the 70 which is now customary. In such colder houses bottom heat was beneficial in order to raise the temperature to 70."
Good point, altagardener. I think the reason the heat mats work so well is that they reduce the range of temp fluctuation in the soil. A few years ago I was starting my tomatoes (early February) in the GH when a friend walked in and said "Hey, start a tray for me", so I did. Everything was the same in both trays, except there was no room on the heat mat for both trays, so one had to be off the mat. Guess which tray was off the mat?? The germination rates were different for the two trays, which were side-by-side, one on the heat mat, one not. My tray had 59/60 germination on the heat mat, his had 52/60 @ 20 days. Not a big difference, but a quick non-parametric test (just out of sheer curiousity) suggested it was significant. Early growth rate also appeared different, but I couldn't quantify it. The heat mat plants were "stockier" and "fuller" than their brothers. Not much, but noticeable when the two trays were side-by-side. Ambient temp in the GH is 60 degrees, which I suspect was the temp in the soil of my friends seeds. Just an observation...don't know what it means...but I'm a firm believer in controlling soil temps!
It's kinda interesting... when you think that in a natural setting, you would normally get at least *some* fluctuation - I'm thinking especially the seeds near the surface of the soil, between sun-warmed days and cooler nights... but then, I guess, germination rates are probably not as high, naturally.
I have seen plants root produce new roots in 2 to 3 days on a well heated pad. It is amazing how well some plants respond to the extra radiant heat. I just recently propagated some Elephant ears on my new mat and after cutting the tubers and dusting them off with sulfur. I am already seeing new eyes form and roots growing out in 3 days. I hope to see the first signs of leaves in 1 to 2 weeks. This is not possible with out bottom heat.
Drywall (gypsum) is a good, cheap insulator that is thinner than styrofoam, tolerates much higher temps without melting, not UV sensitive, and not at all flammable. In fact, it is used in construction to make wooden houses burn a little slower.
P.S. My work place just threw out (recycle bin) some very big steel shleves, painted gray so that they might not need further electrical insulation. Perhaps 24" x 40" .
I thought they would be great "heat spreaders" for this kind of mega-heating pad. They could rest directly on the heating cable, and the trays could sit flat onm the steel shelves. They woujld spread out the 90 degree hot points into some lower-temperature but very uniform heat source.
What luck! What do you usually grow using the heat mats? I have been planting seeds around January for our Master Gardners Sale. This year we are aiming for March which means I will start my seeds earlier. December isn't a very cold month for us. The coldest months are Jan., Feb. and March with the rains.
I tried using a heating pad that was old for propagating seeds, but didn't have any way of knowing what the temp was. Last winter I tried cardoon seeds, (I've never tried that before), Lobelia, vitex, etc. I have been fighting mealy bugs along with others here in La. which put a damper on things.
I did try hydrophonics which I felt was amazing. This would also have to go on the heat mats, but the success rate was much higher. I kept it indoors to watch everything. Small seeds didn't do well; larger seeds did best, but cuttings did wonderful. The cost was about $25 for the tray with floating foam and the soil looking inserts were about $15. All you had to do was fill the tray with water when empty. It was recommended to use ginko bat doo with extras all wound into a cone shape which fit into the styrofoam plate holes. The styrofoam fit intp a plastic tray that held the water. It's about 70 spots for propagation.
For the first time, I had success propagating dwarf junipers. The trick is to leave a heel when tearing off small sprouts to propagat./ The great thing is, it could be left out in the cold all winter long.
Next I'll be planting poppy seeds outdoors, echnicia (cone flowers), propagating dwarf yellow lantana, red tall lantana, etc. I try propagating anything I can put my hands on.
I am still trying Japanese Maples and bottle brush tree/shrub without success, but won't give up.
By the way, I don't work with vegatables. I depend on friends for that.
excellent discussion..does anyone have thoughts on heating large
pots..for my amorphophallus titanum.. i have a heat mat..but the pot is
BIG.. 30 gal.. so very bottom of pot gets the heat..but id like to distribute
some heat up into the pot too..ideally id like soil around 75F
i keep heat mat hooked to a thermostat..at 90F..only the very bottom
of the pot gets that kind of heat..but it moves up
Heat rises, but if there is any air movement, it blows away faster than it rises.
If you can build some kind of enclosure around the pot, sealed at the top, the air space within the enclosure would hold the heat that now blows away or rises and is lost. A heat mat only puts out a few watts. For that to tranlate into 10-15 degrees, you can't afford to let much of it escape.
I would set the pot on top of a square of drywall inside a heavy plastic bag, and wrap the heat mat around the lower half of the pot, inside the bag. Tie or tape the bag tightly to the top of the pot. Hopefully, 90 degrees doesn't melt the plastic!
If you can nail 4 or 6 pieces of drywall around the pot in a square or hexagon, even better. But that obly helps if you can seal the seams (perhaps with tape on the inside) and somehow put a lid on it (more drywall?)
Most important is sealing the airspace around the pot so that each passing breeze doesn't blow all your heat away.
The simplest thing I can think of is to wrap the mat around the pot tightly, then a blanket around the mat, then something waterproof around the blanket.
If you do manage to raise the soil temperature, it might get too hot closest to the mat, and also dry out too fast.
great thoughts here.. much thanks !!
ive been kicking around all kinds of ideas..and tried several
of them on my potted amorphs ..i have my heat mat hooked to a
thermostat..but even with it on 90F bottom of pots are nice and toasty..
but top of big pots hit 64F ..ive wrapped pots with space blanket.. thought of
putting heat cable in biggest pots..but concerned about to much heat..
im going to do some experimenting with enclosure around plants on heat mats..
hadnt considered that..
guess thats what makes us gardeners so interesting..
we try ..try..listen to others..and glean from others experimentation
i know i sure appreciate all here on what others have tried,found what
doesnt work..and what does..
im thinking with my amorphs..ones i overwinter out of soil, then flower,
and pot up..how to get an early start for them.. shy of a greenhouse
with my planted out tropicals..ive been thinking on brians heat mat ideas
why not just use heavy duty heat cables in my beds where i put my
EE's,and bananas.. the small extra electric used..i think will be well worth
the small expense..for maybe a head start with plant growth..
brian.. with your make shift heating system.. im seeing in
many greenhouse/plant catalogs now.. "commercial grade"
and they aint cheap...
heating mats are along your ideas.. ?? did u get in on this??
make $$$ from your FANTASTIC plants..and from your ideas..
well summer is winding down here..and thinkin of "better ways" to keep my
tropical amorphophallus happy campers ..:)
this is what im thinking..
wrapping the pots with heat cables..regulated by thermostat. adhering the cables
to the pots with heat tape..then electrical tape..make sure it stays in place..
i am thinking the heat tape will help keep the insulation against the pot and not
as much heat loss..
what do ya all think???
this would be instead of a heat mat.. thinking this "new way" would distribute
the heat thru the entire pot..not just at the bottom of the pot..
sure would like comments...
I don't have experience with heat cables, so take this with a grain of salt:
Are you sure direct contact won't melt the plastic pot over time? Perhaps there needs to be some metal 'heat spreader' between the cable and the plastic. Several layers of aluminium foil??
I think that to get the benefit of the heat cables, you will have to wrap some heat insulation around the pots-with-cables. Both fluffy stuff like fiberglass batting, and some barrier that prevents the warmed air from just rising and leaving the area.
You could perhaps get even better-distributed heat by making a well-insulated "heat box" of dry wall (gypsum board / sheet rock). Making a base of sheet rock is easy, and four walls not much harder.
You should probably protect the sheet rock from water with plastic film (and arrange some drainage channel.)
However, to keep the heat in where you want it, you would need a cover. Heavy plastic film would keep the warm air from floating right out, as long as it seals somewhat to the pot tops.
Maybe lay down plastic over the tops of the pots, with holes or slits cut for the pots, then drop planks or sheet rock to weigh down the plastic and "seal" it slightly to some of the pot pots.
Just speculation. It sounds pretty Rube Goldberg, as I type it up.
Maybe just lay some fiberglass batting on top of and between pots, foil-side-up, to keep some heat in.
im always grateful to members here..and there ideas/thoughts
good idea on the insulation between pot and cable..but yet able to absorb heat from the cable..
i wont have setting from thermostat on very high..probably.. 80F..enough to put some heat out..
but not so much as to affect the pot.."i think"..LOL :)
your thoughts on box around the pot.. would be good if i was to leave the pot in place..
theres a guy in ohio that grows HUGE palms ..right out in the yard..
hes sent me pics of the structures he puts around them in fall..to protect them during the winter there.. i think hes in zone 5.. gets cold.and lots of snow..
his palms thrive..
for me and my tropical amorphophallus..they will be comming in the house..so..
the soil doesnt have to be that warm..and the house where they will be will be mid 60sF..
i want simplicity..but make my tropicals happy campers..:)
ive thought of just putting a space heater next to them.. just the ambient heat would be of some
help im sure..though the heat will disperse thru the room not just where the plants are..
its all part of what makes gardening fun.. experimentation..:)