I don't think plants are quite as picky about inbreeding as humans. I usually try to plant two of any natives I want to self seed. You won't have as much genetic diversity in your particular yard, but that doesn't much matter when they are decorative plants.

A-hem... I tried to plow through this post and the one with the technically heavy going seed sowing in trays with all the measurement info you posted on the Meadows Forum. Honestly njbiology, I have to say I take a looser, natural, more organic approach to these things and don't get bogged down in process to this restrictive degree, besides, nature will have a say in the end concerning such things as inbreeding (?) which is something I don't worry about so much, besides I personally like the surprises nature always provides. Sounds more like you are conducting a restrictive and controlled science experiment rather than trying to establish native plantings or a meadow. Is this a student science project for school perhaps? Too much of this sort of thing would soon deaden me altogether and suck the life out of what has always been an enjoyable and more flexible occupation. But hey, we each have a different approach.

I treat my yard as a research station.. so in a way, yes. As for edlincoln's reponse: interesting -- but will the seedlings that result from the two plants result in a stable presence of that species in your yard for around 40 years?

Some plants are self fertile and could be maintained from a single specimen. For others you'll need at least two genotypes. Other than that you should be able to maintain a population indefinitely. Franklinia alatamaha is evidence of that. If variety is desired then obviously the more genotypes the better, for example if the goal is to develop new cultivars or preserving alleles of a rare species.

I suppose you could just grow Trilliums and other plants whose natural lifespans are 40+ years. That way you wouldn't have to worry about genetic diversity.

I think one of the reasons you might not be getting the kinds of answers that you may be hoping for is personality/psychology related rather than plant ecology related. Most of the gardeners I know are putterers - we like to mess around in our gardens, adding things, moving things, etc. You've described the opposite of that. Also, most gardeners I know appreciate the dynamic aspect of nature. Most of our property is woodland, and my family enjoys the natural progression - trees fall and rot, small trees grow larger, etc. We have fond feelings for the oaks out front that now shade the only possible place for a veggie garden enough we won't be able to grow tomatoes much longer. Because we enjoy watching things change, I can't imagine managing nature so closely we'd be worried about genetic diversity. But because you sound sincere, I've been trying to think about your idea.

Climate change is an issue. To have healthy plants 4 decades from now, I wouldn't plant anything for which zone 7 is the hotter end of its range - who knows what zone you'll be in 40 years from now.

Other factors that might cause change include your neighbors - our closest neighbor clear cut their yard a few years back. Fortunately, they left a small section of trees on the side facing us, or they would have even more significantly changed the light in a part of our woodland close to our house. Pests also can cause big changes. The Emerald Ash Borer and other pests are getting closer and closer to my area. Future problems with invasive non-native pests aren't something you can predict.

Also, not all plants are good at naturalizing, or self-sowing. Are you planning to collect seed and grow specimens to replace ones that die? If you're not planning on helping things along, you're going to have to make sure you plant only good self-sowers. I don't know what kind of landscape you're envisioning, but some of those kinds of plants are good at getting seedlings pretty far away from the parent plant. You could move errant seedlings, but it sounds like you want to stay away from "hands on management." If you are planning on helping, and for the 10% area if you do plan to use cuttings and vegetative propagation, you'll need a nursery area.

I also don't understand why you wouldn't have more than one of a species even if the population won't be enough to sustain itself? Look at any landscaping book - people tend to like the look of groups of plants, not single specimens. Unless these singletons are trees or shrubs, it might be harder to make things look good.

Whups, no matter how hard I try, I seem to be thinking of reasons why your idea is a problem rather than ideas to help you, sorry! Why not be curious and just try it and see what happens? Surely you'll find that some of your 10% species work for your plan. Some won't, and you can replace those along the way. It's not like you'll be on a spaceship to Mars - it'll be possible to get different plants in 20 years. If you do go ahead with your idea, it would certainly make a unique article in a gardening magazine 20 or 30 years from now.

Good luck! And I wasn't joking about Trillium - plants with longer lifespans would help your proposition work.

I'm pretty sure there are some invasive species which are quite common for which all individuals in North America trace their ancestor to a couple individuals. Particularly in the case of some ornamental trees that are all descended from two or three mass-cloned individuals popular in the garden trade.

You have to ask yourself why you need enough individuals. There are three reasons:

1.) Plants that aren't self fertile. Some plants have male and female individuals. Some don't produce male and female flowers at he same time or have elaborate auto-immune mechanisms to prevent self fertilization. Two plants is enough to prevent this from being a problem.
2.) Inbreeding causing birth defects. Most fatal genetic diseases are recessive. This is party because terrible genetic diseases that are caused by dominant genes quickly weed themselves out of the population. Recessive genes that cause terrible diseases become less and less common until it is wildly unlikely you will mate with someone with the same bad gene. This means marrying a relative slightly raises your risk of having kids with birth defects. All bets are off if you mate with a relative. I've never hear of this being a problem with plants, although logically
3.) Genetic diversity gives a species more options. In a diverse population every individual has a slightly different immune system that is a little better against some diseases and worse against others. In a population without genetic diversity, a diseases that affects one individual will affect all. Also. a diverse population will have some slightly ore cold tolerant individuals, some more salt tolerant ones, etc. This helps them not get eradicated by environmental changes.
In a stable environment, a homogenous population of plants can survive decades or centuries

As someone else said, most gardeners are always trying to tinker with their garden, so they will probably replace plants long before lack of genetic diversity becomes an issue.