Hey Sandiegojane,
Welcome to the forum! I have never tried external heat sources for outdoor Mangos but I was able to inhibit flowering and promote a vegetative flush in winter with grafted trees moved into my garage and supplemented with bottom heat via large seedling heat mats and artificial lighting.
In this thread, I have previously mentioned that planting seeds from Polyembryonic varieties like Sweet Tart, may be a good way of growing mango trees in marginal climate areas. By growing seedlings which lack the mature, florigenic hormones, the tree will grow vegetatively for several years before flowering.
Early flowering causes droopiness in the branches due to the weight of the flower panicles and droopiness of branches exposes the branches to the sun which can lead to sunburn.
The issue with planting seedlings from Polyembryonic varieties is that you won’t know for sure wether you are planting the clone or the zygotic seedling. Not all seeds from Polyembryonic Mangos will give you multiple seedlings and sometimes you will get multiple sprouts from the same segment of a seed.
In order to get a better chance at selecting a clone, you will want to ensure you get at least two sprouts from different segments of the seed making sure each seedling has its own individual root system. If you plant out both the seedlings, there is a better chance of getting a clone.
This year, I am doing a little experiment where I grafted a mango tree with multiple “true Sweet Tart” scions along with scions from Sweet Tart seedlings. I expect the branches grafted with true Sweet Tart scions to flower this year or next but I’m hoping the seedling scions will be delayed in their flowering.
If there is any noticeable difference in the time it takes for first blooms to appear between the mature scions vs the seedling scions, I may be able to graft up trees that will grow slightly better due to delayed flowering/increased vegetative growth.
The rootstock tree has flowered before so I consider it mature rootstock even though the rootstock tree was only about 2-3 feet tall when I topworked it.
This rootstock tree is the same tree as my Double Stone Graft Lemon Zest but one rootstock died so it is a single rootstock tree now. I topworked 15 branches with mostly true Sweet Tart scions but I also added Peach Cobbler, Cotton Candy, PPK, Orange Sherbet Seedling #1, Sweet Tart Seedling #1 and Sweet Tart Seedling #2. This tree was originally Lemon Zest so there are a couple true Lemon Zest branches on this tree and all the grafts are grafted onto Lemon Zest as an interstock between the rootstock and Scions.
This tree was grafted on 9/19/19 and 14 of the 15 grafts have pushed and the last graft is still green but we are about to hit our cold weather so it’s not looking good for it. I reached a low of 44F early this morning.
Simon