University of Vermont Extension 
Department of Plant and Soil Science

Spring News Article
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SHAMROCKS FOR ST. PATRICK'S DAY

Dr. Leonard Perry, Extension Professor
University of Vermont 
 
You don't have to be Irish to wear green on St. Patrick's Day, nor do you need a green thumb to grow shamrocks indoors. This plant, which is associated with this March 17 holiday, is quite easy to grow.
           
Shamrocks are a member of the Oxalis (wood sorrel) family, which contains more than 300 species. Most of these grow from small bulbs although some have tuberous roots. The distinguishing characteristic is the three rounded or triangular-shaped leaves at the end of delicate stems. Most oxalis plants fold up their leaves at night, hugging them tight to the stems until daylight "wakes them up" again—described botanically as “nyctinastic movement.”  Due to oxylates in plants, they may be toxic to cats, dogs, and horses if ingested in large quantities.
           
The familiar St. Patrick's Day species (Oxalis acetosella) is available at florist shops and many grocery stores this time of year. It has tiny, dark green, triangular leaves and grows to a height of about six inches. This variety hails from Europe, Iceland, and Asia. It is not the official Irish shamrock (Trifolium dubium)-- a yellow-flowered clover or trefoil-- which is what the majority feel is the authentic species. That clover is difficult to grow indoors, so nurseries and florists sell Oxalis plants instead.
           
If you are wondering how the shamrock became part of Irish history, there are many explanations going back to the significance placed by the ancient Celtic peoples in the number three. The most popular story is that St. Patrick, who is credited with bringing Christianity to Ireland in the second half of the fifth century, once plucked a shamrock from the grass at his feet to illustrate the doctrine of the Holy Trinity to his congregation.  St. Patrick's Day, which is celebrated on the anniversary of his death, also heralds the arrival of spring. The shamrock was adopted as a symbol of both this patron saint and the "season of rebirth," and can be seen on ancient coins and medieval tombs.
           
Shamrocks like cool air, moist soil (except in their dormant period), and bright light. Ideal temperatures are between 50 and 65 degrees (F), and over 75 degrees may cause plants to become dormant.  Soils should not be kept too wet or waterlogged.  If the plant yellows, it may be getting too much water and roots may rot.  Too little water and it obviously wilts. Too little light, or too much warmth, and plants may get tall and lanky.      
           
They do not have an extensive root system, so unlike many plants, actually prefer to be crowded in a pot. However, if the plant dries out too quickly, you may need to move it into a larger pot. Fertilize every two to three weeks while the plant is actively growing or flowering, using regular houseplant fertilizer. For application rate, follow directions on the container.
           
Be aware that no matter how much care you give this plant, at times it will look sick and lose its leaves. This dormant period, which often occurs during summer, is part of the growing process common to all plants grown from bulbs.  During dormancy, stop watering. Let the leaves die back naturally, then remove dead, brown leaves. Place the plant in a cool, dark place while it goes through its dormancy period. Plants generally "sleep" for about three months. New green shoots signal that the plant is waking up and needs to be moved back into the light.
           
To divide your shamrocks, wait until the bulbs reach the end of a dormant cycle. Take them out of the pot, and remove small side bulbs. Then replant, just under the surface, in a mix of potting soil and sand. Place in a non-south facing window. Water, keeping the surface just moist to the touch, until plants become established.
 
Other Oxalis varieties have similar growth requirements though many go through a shorter, or no, dormancy period. Depending on the variety, the plant may have yellow, white, pink, purple, or red flowers and grow as tall as 10 inches. Leaf color ranges from dark green to deep red.  Those with dark red or purplish leaves only need about a month of dormancy.
           
Whether you are giving or getting shamrocks this St. Patrick's Day, there's one more thing to keep in mind. Because these plants go dormant, shamrocks are not suitable for growing with other houseplants in mixed pots or planters. If you get a mixed basket of plants, after the holiday, separate the shamrock from the rest and replant in its own container. Don't wait until the plant turns yellow or sickly looking. By then, its roots will be deeply intertwined with the roots of other plants and will be difficult to transplant. 
           
By giving your Shamrock plant bright light, even moisture, and cool temperatures, and letting it go dormant during summer, it should provide years of enjoyment.
 

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