They will likely not survive without some protection. Actual soil temperatures at 6" depths rarely drop below 25*, even in zone 4, but containers sited above ground can see temperatures as low as or nearly as low as the air temperatures, which is far too cold for tulip bulbs. If I was you, I would rethink my strategy and find a way to get these bulbs in containers below ground or on the floor of an unheated garage or other building that is weather-tight.

Al

tapla is right they wouldn't survive the winter in the planters. The freezing and thawing would kill them.

Do you have a place where you could sink pots into the ground? You could plant your tulips in deep pots. The bulbs have to go 6 inches under the soil up to the brim of the pot, or you could go 4 or 5 inches and give them a 3 or 4 inch layer of mulch on top. Dry leaves work very well as a mulch.

About a month before your event clear off the mulch and lift your pots and plant them in your planter pot and all. When they are finished blooming you can lift the pots and plant something else. Set the pots somewhere in the sun and wait for the foliage to die back. Pull off the dead foliage and store pot and all in a cool dry dark place. Don't water. Replant the bulbs in the fall in your garden.

Plants that are genetically programmed to withstand chill to a particular low temperature (not considering cultural effects on cold-hardiness) can freeze/thaw innumerable times with no ill effects, as long as the temperature does not drop below the genetically determined killing low.

IOW - a plant able to withstand root temperatures down to 20* can see the soil frozen solid repeatedly with no ill effect as long as the temperature remains between 21-42* (cultural effects other than chill not withstanding).

Al

Yikes! Thank God I asked! Ok, re-thinking the entire plan. The planters are way too heavy (with dirt) to move from the porch, so I will have to plant all the tulips in separate pots and place them in the planters in the Spring.

One more question though: We have a really bad chipmunk problem and I would rather not keep the bulbs in the ground. Would it be feasable for me to plant the bulbs in terracotta pots and store them in my attached garage over the winter? (I am in zone 5--Cleveland.) If this is ok, then onto the last question: Will they bloom sooner than they are generally expected to? (Triumphs are mid bloomers--April to May... so should I expect them earlier than that if they are overwintered in pots?)
Thank you so much for taking the time to answer---you just saved me from being heartbroken come Spring!

My planters are about 5 ft. long by 1 ft. tall. If I just buy those plastic rectangular liners, plant the tulips in there and store them in the garage, that would be easier than pots. BUT, I can't find liners that are taller than the standard 5 1/2 inches. Will this be enough depth to plant tulip bulbs? How deep do they need to go? I read a different answer every time I research it.... Ugh.

"... plant the bulbs in terracotta pots and store them in my attached garage over the winter?" Yes. I've stratified and over-wintered dozens & dozens of containers filled with assorted bulbs in the garage. You could also buy pieces of 1/2" hardware cloth and lay them on top of the soil to thwart the chipmunk's digging, but you still need to keep tabs on the temperature where you will over-winter.

"Will they bloom sooner than they are generally expected to?"

It depends, but once the bulbs have stratified (been exposed to an appropriate period of cold temperatures) they are capable of growing & blooming after several days of temperatures above 45*, so it is almost certain that they will be ahead of their counterparts in the landscape. I over-winter about 100 (bonsai) trees in my garage and typically, they begin leafing out and growing at least 3 weeks ahead of their counterparts in the landscape. Additionally, don't forget that after stratifying (mid-late Jan) you can move containers indoors where you can enjoy them when they bloom after 2-3 weeks. You can also do this (forcing) with branches cut from many spring-blooming trees in the landscape (apple, pear, forsythia, pussy willow, ...........)

Good luck.

Al

I beg to differ tapla. eg. My friend planted 2 cedar trees in a large planter. That winter was a mild one here and the temperature never went below -25C all winter. In other years we can get to -40 C and our cedars in the ground do just fine.

Both trees died that winter. Can you offer any other explaination why, except the repeated freeze and thaw. I'd be willing to listen

Thank you for the response. Will the 5 inch liner (that I would keep in the garage and then transfer outside in the spring) be deep enough?

Also, I should have added that these planters are pretty big.... made from cedar and are 5 foot long by 1 ft. wide and 1 ft. tall. They will require a LOT of dirt to fill. Do you think with so much dirt that the tulips WOULD be ok in them outside over the winter? Just wanted to make sure you knew they weren't little pots I was talking about if that does make any difference... (again, novice that I am I have no idea!)

I'm certainly no stranger to being disagreed with, but to conclude that because two trees died and they had it must have been the freeze/thaw cycles is not logical.

Consider the logic, which mirrors what you are saying: Your friend Joe is scratched by a cat while visiting another friend. Two days later he comes down with a fever, then dies. You conclude he died from cat scratch fever, never stopping to consider that it could have been any one of hundreds of other maladies.

In the case of the trees you must consider other possibilities and have some means of eliminating them before you adopt a reason that is contrary to known science, which would be a considerable uphill battle.

Other possibilities for the demise of the trees:
Too wet
Too dry
The roots were exposed to killing low temps
Not enough energy reserves going into winter
Insect infestation
Disease
Others

Besides - you relate an anecdote about two trees that died from unknown causes, and an experienced grower relates that >100 trees he overwinters in an unheated garage which see repeated freeze thaw cycles, not to mention the several trees (only the hardiest pines & junipers) that remain outdoors above ground all winter and that see dozens or repeated freeze/thaw cycles do NOT suffer ill effects from the freeze/thaw, so logic tells us, even w/o science, that the likelihood of my observation being the more accurate is much greater.

Now - getting to the science ..... this is a copy/paste job from something I contributes to another thread a while back:

Commonly, each species of plant has a general range of cold-hardiness. Within species and cultivar, cold-hardiness is genetically determined. That is to say that a plant that is propagated from cuttings or tissue culture will have the same ability to resist cold as the parent plant. Plants cannot "develop" a greater degree of cold-hardiness by repeated or prolonged exposure to cold, even after 100 years (trees).

If we pick any plant at random, it may or may not be able to withstand freezing temperatures. The determining factor is the plants ability to prevent freezing of bound water. Bound water is the water inside of cells.

There are actually three kinds of water to consider when we discuss "freezing". The water held in soil - When this water freezes, and it can freeze the soil mass solid, it doesn't necessarily kill the plant or tissues. Then there is free or unbound water, also called inter-cellular water. This is water that is found in plant tissues, but is outside of living cells cells. This water can also freeze solid and not kill the plant. The final type of water is bound water or intra-cellular water. If temperatures drop low enough to freeze this water, the cell/tissue/plant dies. This is the freeze damage that kills plants.

Fortunately, nature has an antifreeze. Even though temperatures drop well below freezing, all plants don't die. In hardy plants, physiological changes occur as temperatures drop. The plant moves solutes (sugars, salts, starches) into cells and moves water out of cells to inter-cellular spaces in tissues. These solutes act as antifreeze, allowing water in cells to remain liquid to sometimes extremely low temperatures. The above is a description of super-cooling in plants. Some plants even take advantage of another process to withstand very low temps called intra-cellular dehydration.

The roots of your trees can stay frozen for extended periods or go through multiple freeze/thaw cycles w/o damage, so long as the temperature does not fall below that required to freeze intra-cellular water. If roots remain frozen, but temperatures remain above killing lows, dessication is the primary concern. If the tree is able to take up water, but temperatures are too low for the tree to grow and make food, stored energy becomes the critical issue. Dormant and quiescent trees are still using energy from their reserves (like a drain on a battery). If those reserves are depleted before the tree can produce photosynthesizing mass, the organism dies.

There are a number of factors that have some affect on the cold-hardiness of individual plants, some of which are length of exposure to seasonal cold, water availability (drought stressed plants are more cold tolerant), how recently planted/repotted, etc

No one can give a definitive answer that even comes close to accurately assessing the temperature at which bound water will freeze that covers the whole species. Unbound water is of little concern & will usually freeze somewhere around 28*.
Some material will be able to withstand little cold & roots could freeze/die at (actual) root temperatures as warm as 25-27*. Other plants may tolerate much colder actual root temperatures - as low as 10*. There's just no way of knowing unless you have a feeling for how cold-tolerant the genetic material the plant was derived from might be, and finding out is expensive (from the plant's perspective). ;o) Another example of this genetic variance is that trees found growing and fruiting well closer to the equator need no chill time, while other trees, derived of genetic stock from a more northerly provenance may need a period of chill to grow with optimum vitality in the subsequent growth period/cycle.

It's wise to remember that root death isn't instantaneous at one particular temperature. Roots succumb to cold over a range of chill with cultural conditions affecting the process. The finest roots will die first, and the slightly thicker and more lignified roots will follow, with the last of the roots to succumb being the more perennial and thickest roots.

Since any root death is a setback from an energy allocation perspective, and root regeneration takes valuable time, it's probably best to keep actual root temperatures in the 25-40* range as long as we can when the tree is resting, even though the organism as a whole could tolerate much lower temperatures. Even well established trees become very much like cuttings if all but the roots essential to keep the tree viable are lost to cold. Regeneration of roots is an expensive energy outlay and causes the trees to leaf out later than they normally would and shortens the natural growth period and reduces the potential increase in biomass for the next growth cycle and perhaps beyond.

Sorry for the detour, Joann. The 5" liner will be fine. I use shallow bulb pans almost exclusively. Use a soil that drains VERY well, and try to be sure the bulbs are not touching when you plant.

The volume of soil in the container is an insignificant factor. A larger volume only increases the length of time it takes for the soil to fall to the same temperature as the surrounding air. It doesn't change how low the actual soil temperature will eventually drop to, which is the critical issue. FWIW - large containers resting on the ground do gain some advantage from geothermal heat, but in the case of tulips, it wouldn't be enough to ensure their survival.

Take good care.

Al

Goodness Al, that may be the most helpful, thorough answer I have ever received! Thanks so much again---I will do as you suggest.