Hello, this year our freeman maples produced incredible number of seeds. Surprisingly many rooted and produced seedlings in our terrace pots. I’ve transplanted about 50 into 6” deep planters. My questions: 1) should they be in deeper pots? 2) when can they be safely planted in soil?3) should we bring them inside over winter and plant next spring? 4) the seedlings are from 10 trees planted by the city lining our street, are cultivated maples supposed to be infertile?5) when planted in soil, what care and protection is needed to ensure they grow?
Thank you for contacting the Toronto Master Gardeners about your Freeman maples (Acer x freemanii). This tree is a hybrid – a cross between a red maple (Acer rubrum) and a silver maple (Acer saccharinum). It has a number of cultivars which are more commonly planted than the species. According to the City of Toronto Street Tree Guide, it is the cultivar ‘Autumn Blaze’ that they plant. Interestingly, from everything that I have read, this cultivar has very sparse flowers and fruit – apparently that is not the case this year for your trees! (There are also other cultivars that produce seed.)
Freeman maple ‘Autumn Blaze’ grows to 40-60’ in height and 20-40’ in width. It is very fast growing, about 3’ each year in optimal conditions.
I think the best approach to getting your seedlings in the ground is to plan to plant them next spring, since when the frost arrives this year they will still be very young, the bark on these trees is very delicate and the root systems will likely not be fully established. Assuming that they are likely to be around two feet tall by the time they go dormant in the fall, I think by then they are going to need to be in individual one gallon pots (approximately 6-7” in width and depth). I think it would be best to plan to leave them outdoors (with the proper protection) during the winter vs subjecting them to the shock of going indoors in the fall and then back outside in the spring (and then transplanting them).
There are a couple of ways to protect your plants outdoors during the winter. You could bury the pots in the ground, add mulch to the soil above the pots and then further insulate them by putting a cylinder of chicken wire or other fencing around each plant and fill it with straw. Soil is a good insulator to protect the plant roots. Or you could put the pots in an unheated garage or other building where the temperature is maintained at around -7 – 7C. You will need to keep your plants well watered until the soil freezes in the fall and and then start again when it thaws in the spring. Here is a good article about these two approaches to protecting your plants for the winter.
In the spring you can take your pots out of the ground when the extremes of winter have passed and the ground has thawed so that you can dig and remove the pots. Or bring them out of wherever you have stored them to a sheltered spot and gradually re-introduce them to sunlight. This guide for Planting a Tree for Life by the Toronto Master Gardeners is a good source of information as you prepare to plant your trees. In order to thrive, your trees need full sun (at least 6 hours daily ) and moist, well-drained soil. Their fast growth rate often results in poor structure so it will be important to stake your trees after planting.
‘Autumn Blaze’ requires little care. Most important is to maintain the moist soil around the roots while it’s still young and establishing itself. Mulching will help with this. It doesn’t need a lot of pruning, although it tends to have poor limb structure so some pruning will keep it strong and healthy and not top-heavy. Pruning is best done when it is dormant. Do not prune in the spring because it is a ‘bleeder’ and will lose a lot of sap if pruned at that time. This plant doesn’t require fertilizing if your soil is healthy. The best way to do this is to add compost to the top layer of your soil every year. It prefers slightly acidic soil, so it would be best to do a soil test to check the pH of your soil, and then amend it as needed.
Good luck with your lovely trees!