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Crocuses in SE Pennsylvania

Not native ones, unfortunately.  However, the number of species Crocus grown in this corner of PA has increased dramatically in the past 6 years, following our move from the UK, in 1995, to Exton, Pennsylvania in the NE USA. 

Exton is in USDA zone 6b, winter minimum temperatures can reach -5F, summer maximum is over 100F.  Humidity is very low from fall to late spring/early summer but then often reaches 100% in July and August, accompanying temperatures in the 90-100F range.  Much of the winter can pass without snow cover, approximately 36" being the average total snowfall. Ice storms can be frequent and spectacular.  The number of days with abundant sunshine is great and, consequently, the conditions are excellent for growing 'in character' bulbs, in particular.  In an average week we can expect at least 5 sunny or partly sunny days.  The result is flowers which open as the buds are emerging through the ground.  Four defined seasons is the  norm, fall colors are spectacular and the growing season is very long, generally from February through November.  Indeed, crocuses are frequently the first plants to bloom in the garden, and also the last.


Our garden covers approximately 1.5 acres and is on three levels.  The lower level is generally exposed to the south and hence very sunny, although in places shade is provided by a number of specimen trees.  The entire garden is traversed by a 40 degree slope which corresponds to the woodland edge; the slope is some 400 feet in length and around 100ft deep.  The top of the garden is native deciduous woodland.  The soil is moderately acid and is superbly drained.  In many places the ground is very rocky, with large exposed rocks, some pockets tend towards pure sand.  With the exception of the raised beds, no attempt has been made to modify the soil, and it appears so far that none is necessary.  Crocuses are, of course, gastronomic delights to the many rodents which frequent these parts and maintenance of an 'unprotected' collection would be almost impossible.  However, our five cats do a fine job in reducing the chipmunk, mouse, vole, squirrel and rabbit populations to such a low level that I don't know of any bulbs that have been lost to the wildlife.

Having spent the 10 years prior to our move growing a wide range of alpine plants and bulbs in pots under glass, I wanted to take advantage of our new-found garden and try to make the move to 'proper gardening', keeping in pots only those plants which absolutely need it.  Thus, whilst I have two greenhouses, these are used only for raising bulbs from seed prior to introduction into the garden, and for growing plants which may be 'tender', and have not yet been attempted outside.    The smaller greenhouse (twin-wall polycarbonate) is used mainly to hold pots of ungerminated seed.  Affording protection to seed pots has resulted in a significant increase in success raising crocus from seed.  Seeds of many crocuses germinate during fall and winter but it can be a number of weeks before anything above ground can be seen.  During this period I have found them to be very susceptible to freezing, especially when wet.  Lack of success attributed to germination failure was, in fact, due to death of the immature seedlings.  Maintenance of the greenhouse temperature no lower than a few degrees below freezing has overcome these problems.  As soon as germination is evident, the pots are moved into the larger greenhouse.  The latter has a twin skin, the outer being acrylic sheet, the inner a ridged acrylic polymer which traps a large air space between the faces.  There are no windows but two large exhaust fans and corresponding intakes facilitate excellent air movement and keep the temperature only a few degrees above ambient.  The fans are thermostatically controlled and on whenever the temperature rises above 45F.  Natural gas heat keeps the minimum temperature just above freezing, the heater switches off around 42F.  Seedlings stay in the greenhouse until they are approximately flowering size, being repotted in their 3rd year if necessary.  The only crocus with a permanent greenhouse home are the tender species such as Crocus aleppicus and Crocus moabiticus .  However, a friend in Oregon reports that the latter has survived (and thrived) in a cold frame in which temperatures have dropped well below zero.  Stocks at the moment don't yet permit risking a replication of this feat in PA! 

The remainder of my crocuses are grown outside, either in the ground or raised beds at the woodland edge, or in deep sand beds.  Crocuses grown at the woodland edge include many of the normally recommended candidates for cultivation outside in the UK – for example C. banaticus, vallicola, nudiflorus, gargaricus ssp . herbertii, kotschyanus ssp. kotschyanus and ssp. suworowianus, tournefortii, speciosus forms, longiflorus and goulimyi.  I have tried a very few C. pelistericus and scardicus in similar positions but these have failed to do well, only the former still lingering.  It is likely that the summer heat is the problem, it is very hard to keep them uniformly moist throughout their long growing season.


All other crocuses are grown in beds of pure sand, results to date have been excellent, with a large number of species thriving.  Beds are made from treated landscape timbers (8' long  x 5" wide x 3 1/2" high, $3 each) anchored into the ground with 2' lengths of rebar (concrete reinforcement).  Three layers of timbers generates a bed approximately 10" deep, sizes of beds varies from 16' x 8' through 24' x 4' to 24'x 12', the latter in three terraces.  They are remarkably easy to make, and very stable and long lasting.  The sand I use is a children's play sand, clean and sharp, with an average particle size around 1mm, costing $25 per ton, delivered.  It facilitates excellent drainage, especially when combined with the sandy soil upon which the beds sit, yet does not dry out very quickly either.  The secret to extended winter hardiness, in the case of many bulbs, is to avoid rapid freezing in wet conditions, especially pots.  The sand beds behave such that many bulbs will survive at temperatures well below the accepted norm, even though they freeze completely through for extended periods (the longest to date is 4 weeks during which the temperature didn't rise above freezing and dropped to around 10F at night).  For example, Iris kirkwoodii , thrives under these conditions, two clumps this year producing over 20 flowers each.  Crocus imperati ssp . suaveolens was frozen part way through her display and encased in ice for over 2 weeks, but continued as if nothing had happened when the ice thawed.  Many of the beds are unprotected, a couple are covered with 8' x 4' twin-wall polycarbonate sheets, 12" above the beds, from June until September.  A very wet summer in 2000 caused rotting of a number of Corydalis and the protection is intended to guard against this in future.  Unlike the corydalis, the crocus were not affected by the relatively damp dormancy.

Crocus which leaf out in the fall can suffer significant leaf damage over winter, although the corms are fine and vegetative increase is not affected.  Perhaps surprisingly, it is not the cold per se which causes the damage, rather the effect of snow or ice on the leaves.  Although snow is possible as early as late-November, it is rare before Christmas and the majority falls in January and February.  Temperatures as low as 5F cause no apparent damage but ice lying on leaves causes yellowing back from the tips; by winter's end as much as two thirds of the leaf can be damaged.  Crocus goulimyi seems particularly susceptible but I seem to get far more concerned than the plant!

In summary, I have been surprised and delighted at the wide range of crocuses which thrive outside in the conditions described above.  I am sure that crocuses, like many bulbs, grow better in the open ground or raised beds, than in pots, and it is well worth experimenting to try to find conditions outside to their liking.