I found a green acorn that had just fallen off the tree – I want to sprout and plant it , but I think I’ve waited too long because now the acorn has dried and is brown and it floats. I’ve read that a floating acorn is not necessarily unviable , although it may be. Is there a way to rehydrate the acorn so it will germinate? Should I store it over the winter and plant in the spring? Could I plant it in a pot and keep it in a sunny window over the fall/winter? Thank you!
Thank you for contacting the Toronto Master Gardeners with your inquiry.
According to an article entitled: Test of the Float Method of Assessing Northern Red Oak Acorn Condition published by Linda S. Gribko and William E. Jones in Tree Planters’ Notes
“The float method of assessing acorn condition has been widely recommended as a fast, inexpensive, and nondestructive means of differentiating between sound and insect-infested or otherwise damaged acorns (Korstian 1927, Schopmeyer 1974, Stockton and Morgan 1979, Bonner and Vozzo 1987). Batches of acorns are placed in water; those that sink to the bottom are deemed viable whereas those that float are assumed to be insect-infested, aborted, malformed, or diseased. Although true viability of the embryo cannot be assessed using the float method, it may be used to differentiate between sound acorns and those that have a reduced probability of germination.” -Tree Planters’ Notes Volume 46 N0 4 https://rngr.net/publications/tpn/46-4/46_4_143_147.PDF/at_download/file
Acorns should be planted as soon as they are collected since they will lose viability as they dry out, if they are not planted immediately they should be stored in a moist cool environment. Moisture causes acorns to germinate; refrigeration of the acorn will slow down their germination.
Acorns are best harvested in the early-to-mid autumn before they have fallen from the tree. Check the acorn for insect holes, softness or mold. Once you have a viable acorn place them in a ziplock bag containing damp paper towel. Place the bag in the fridge for a month and a half or longer. This process is known as stratification, which is simply exposing a seed to cold temperatures, mimicking the natural conditions that a seed would experience had it fallen to the ground. Make sure to periodically check on your acorn for rot or dryness. Most acorns will germinate in the presence of moisture. Whether or not the root has cracked through, the acorn is ready to be planted in a pot filled with good quality potting soil after about 40-45 days of storage. Discard acorns with discolored, soft, or mushy roots. Place your seedling in a south-facing window, making sure to water frequently.
When planting your acorn in your garden be sure that you have selected a site that will be large enough to accommodate the tree’s mature size. Oaks require sunlight to survive, so don’t plant them in shaded areas. The following link is an excellent article with step by step instructions on on How to Plant an Acorn and Grow an Oak Tree
Here’s to enjoyment of your future oak tree.