I have a candidate: It is a micro dwarf that might do ok in 6" deep soil.

HAHMS GELBE TOPFTOMATE

It has a long German name for such a tiny tomato plantIt grows about one foot tall and bears yellow cherry size tomatoes. Here is a picture:

It is cherry size blue tomato plant. But this one can grow almost 3 ft tall. You might be able to find it sold at the BBS.

Sey

"The raised beds are directly over the blacktop - so only about 6 inches of dirt."

That is going to be their main source of problems - the heat from the asphalt will cook the plant roots and only 6" of soil isn't nearly deep enough. Doesn't bode well for any success regardless of the variety used.

Difficult to find any of the dwarf varieties for sale as transplants so you'll very likely have to grow your own from seed. Is that going to be possible this late?

Dave

I would advise buy some 5 gal sized self watering containers and Bonnie Plants Patio tomato plants. Also Tumbling Tom is a prolific cherry plant that stays small. Place the pots in the garden area.

It might be worth just trying as planned. I'm not sure that the covered blacktop will heat or that the conducted heat will be that significant. You didn't mention your daughters age but this could be a learning experience for kids to measure soil temperature and ambient temperature and note plant performance.

Note that many dwarf varieties are relatively shortlived compared to even determinate varieties. I have Microtom plants with ripe fruits now in pots but just looking at the plant it is evident that aside from a few lower branches on the 6" plants the upper plant is all tomatoes and it stops there.

My recommendation would be to try a small fruits determinate variety like Mountain Magic. That variety has good disease resistance and would handle more stress than many other varieties.

i think on covered black top they would at least have to be 1ft high or better 18 inches. dont think i'd even attempt 6 inches on asphalt.

Don't know where the OP is located but down here asphalt easily reaches 110-120+ degrees in the sun. When you can't even walk on it you sure can't grow anything on it in only 6" of soil. To make it a beneficial learning experience for the kids why start out with 2 strikes against you?

Dave

You might consider a higher raised bed. Just adding six inches of soil to what you have might make it more possible. It would also improve drainage. I'm less worried about asphalt. Asphalt gets hot because it is black and it absorbs all the sunlight. If that asphalt can't see the sunlight because it is buried under soil it's not going to heat up. Now, if exposed asphalt is near where the plants are to be grown, that might be a worry.

Like I said try it since nobody really knows the root zone temps. My tomatoes have black plastic covering every square foot of soil and it gets very hot to touch on any sunny day but I've done it for years with success.

Unless I misread what the OP described, the root zone temps ought to be just fine. The raised beds are "directly over the asphalt". Which means that the sun isn't hitting the asphalt. If the raised beds are propped up an inch over the asphalt, it *still* wouldn't be a problem, because the asphalt is shaded.

The main problem here is the soil depth. I suspect the plants would grow in siz inches of soil, and I suspect they might produce some fruit, but in no way would the plants mature as large as a properly bedded plant would.

You may be right Dan. I interpreted it as the beds sitting right on the asphalt - ie: placed directly on the school parking lot asphalt - rather than raised above it. And asphalt is a good transmitter of heat so if the surround asphalt is hot the asphalt under the bed could still be overly warm and that higher temp would be transferred to the soil. But without more info from the OP it is likely a moot point.

Dave

The school is in zone 11 a. So even in August September and October we only hit around mid 70s. I was looking for something that would be pretty low maintenance once it was set up - there won't be much staking, pruning, etc. and hopefully once the tomato is in the only thing to do would be to water it once every couple of days. It's ok if there's not a ton of fruit as long as there is some fruit so the kids can see what happens from seed to fruit. I will just supplement with some home grown or store bought tomatoes for any projects we'll need. Thanks for the advice. I'll see what I can find in the stores and online later today.

Dave, that's a fair point. If the bed is small, and it's laying on asphalt, of which the surrounding asphalt is at roasting temperature, some of that heat will get conducted under the bed. The air temperature will be high as well.

As to low maintenance, if this is a proper container garden, you're going to be needing to regularly fertilize as well as water. But with only 6" depth, you're pretty much guaranteed not to get a lot of fruit.

Uh, zone 11a? Is this Hawaii or Puerto Rico? Or somewhere else in the tropics? Summer temps peak in the mid-70s? Something not adding up here. High altitude? I'll say it once again. USDA Hardiness Zones don't mean anything useful for a summer gardener.

"USDA Hardiness Zones don't mean anything useful for a summer gardener."

Well it tells us he/she isn't in the continental US. ;-)

But asphalt is still asphalt and 6" is still only 6" no matter the location.

Dave

The school is by the ocean - a city called Playa Del Rey.

If that's in coastal LA, the hardiness zone is 10b. The mean max temp in summer (downtown LA at least) is in the mid 80s. Could be lower at the beach. Of course, that's very different than more continental 10b sites that get more violently hot in the summer. Again, USDA hardiness zones tell you little about summer conditions.

But yes, hot asphalt is probably less of a risk there that we were figuring. But I really think you'd help yourself out enormously by somehow adding a few inches depth to your bed. Say, you might slap some white paint on the asphalt near the bed!