Often the most rewarding greenhouse will be one you’ve filled with plants you’ve grown from seed. It’s that personal connection, you’ve known them at each stage, rather like your own children. And if you have children little will give them such an appreciation of nature as growing plants from seeds in fruits they’ve eaten.
There can be few of us who’ve not tried to grow an avocado stone, lemon or apple pip. Sadly too often such enthusiasm gets blunted by unwitting error. Unfortunately many seeds are unlikely to grow in ordinary soil. Especially if it’s permanently wet, or ever allowed to completely dry out. Most seeds do best sown in proper Sowing compost, this has less nutrients than Potting or Multi-purpose so as not to over-load germinating seedlings.
Next most seeds germinate only when they’re warm enough, then thrive only if light enough, so a greenhouse is usually far more successful than a windowsill. Naturally a frost free or warm greenhouse is preferable but a small heated propagator can give excellent results for less to install and run. (Though you will want to heat your greenhouse once you’ve many gorgeous plants getting big enough to display.)
Now to avoid false hope it may help to know which exotic fruit seeds will produce interesting plants, and also whether you can actually hope for fruit.
So the following are three groups that will likely germinate and grow into perennials suitable for a warm greenhouse.
The first are all great fun but frankly are unlikely to ever crop well if at all for you: Avocados! Loquats, Dates, Mangoes, Tamarinds, Papayas, Custard apples, Guavas, Carob beans, Kiwis and Dragonfruit.
Fortunately with some others you really can expect to harvest, given time: Citrus, may even come true but you need serious patience, possibly 2 decades, Feijoas about 10, Apricots take only 5-6 years and come fairly true as do Peaches & Nectarines and in the same time, Pomegranates and Grapes take 6-7, while Passion-fruits and the speedy Cape Gooseberries just a year or two.
For those with less patience there are of course those standard tender annual crops sown in late winter: Tomatoes, Melons, Watermelons, Chilli & Sweet peppers, Aubergines and Okra.
However for kids of any age I strongly recommend the very best seed to grow from a fruit is a strawberry, wild, Alpine or cultivated as these plants fruit in just a year or so in cool greenhouse conditions, take relatively little space and are always truly delicious.