Q. I have always loved Mimosas and assumed they grew from flowers until I saw the awesome trees in southern France. I started some Mimosa trees from seed, and they are now about 3 feet tall and becoming a problem to move inside for the winter. I don't think they are hardy here in Pennsylvania, but thought I'd check just in case I could get away with planting them in the ground if I mulch them heavily and wrap them in burlap for the winter. If not, I need to repot them and was wondering if our neighbor's aged horse manure is too strong to add to the potting mix. They have done quite well for us without any fertilizer.
A. If your plants are Albizia julibrissin, they will grow outdoors in Western Pennsylvania. If they are one of several species of mimosa, they will not be winter hardy here. Both plants are in the pea family and commonly known as mimosa. Although Latin names can make people trip over their tongues, these names are the only way to know for certain that we are talking about the same plant. Albizia julibrissin is also known as Powderpuff Tree or Silktree.
Silktrees are native to eastern and southern Asia. They have become naturalized in the United States from Maryland south to Florida, and west into eastern Texas. They do grow well in our climate but may be killed back to the ground in severe winters. Even if a severe winter kills them to the ground, they generally re-sprout from the roots. Their seeds frequently germinate in the soil under the tree or in adjacent flowerbeds, so you usually have seedlings coming up just in case the mature tree dies.
Silktrees prefer full sun and well-drained soil but are very tolerant of poor, infertile soil because they are in the pea family. Such plants are known as legumes, and they have the unique ability to fix nitrogen directly from the atmosphere. Silktrees can be troubled by vascular wilt and mimosa webworm and are often thought of as short-lived trees.
They are classified as an invasive alien species in areas where they have naturalized in the United States. They spread by sprouting from their roots as well as setting prolific seed crops. Since they are able to grow in very poor, dry soil, silktrees quickly colonize areas and squeeze out native plants that provide food and shelter for wildlife. They do seed themselves in our climate as well, but I personally have not seen them grow so prolifically that they threaten local ecosystems. Please be aware of their potential weedy character and grow them with care.
If your tree turns out to be a mimosa, you will have to keep it in a pot and move it into an area that does not fall below freezing for the winter. The sensitive plant, Mimosa pudica, is often grown in high school biology classes for students to experiment with plant growth hormones. They grow quickly and they hold the students' interest because their leaves close up and "wilt" when you touch them, but they recover quickly. They can be fun to grow as houseplants and can grow to 5 feet tall with a slightly smaller spread.
If you need to keep your "mimosa" in a pot, just move it up one pot size. Moving it into a much larger container can lead to root rot because the large volume of soil (compared to the size of the plant) holds too much moisture for too long a time after you water. You can use a small amount of your neighbor's aged horse manure to amend the potting mix -- roughly one-quarter of the mix by volume. Manure can be high in salts that can damage many plants. This is especially true in a container where the small volume of soil cannot buffer the salts as well as the much larger volume of soil in an outdoor planting situation.