Along the same lines I field questions:
...how many cabbages do I get out of a pack of seeds?
...how many dinners can I get out of a pack of bean seeds?
...how much more vegetables will I get if I use store bought fertilizer? Or if grown organically?
These are impossible questions to answer, though the questioner simply can't understand that. And I sometimes have a hard time getting them to see why there is no easy, simple answer. I've had a number of people leave in frustration, feeling that I know nothing about gardening because of my non-decisive response. My replies are honest. I'm not trying to impress anybody nor am I trying to look the part of an expert. But that makes little difference. People want answers, NOW. I'll often suggest doing a google search. There are charts on the Internet that will give you an answer.
So why my wimpy stand? Why not give a knowing answer like 23 broccoli plants, 19 1/2' row of peas? Lets explore why.
First and foremost is the skill of the gardener. If a person is new at a job, they are not going to be very good at it. Just think back to when you learned to ride a bike! Not so good, eh? When I first started to garden I couldn't get much out of the garden even though I was thrilled and proud of what I did get. I thought I was quite the gardener! New gardeners often have scrawny little beets, small cabbages, few potatoes, that sort of thing. As they learn to feed the soil, regulate moisture, control weeds, then their quality and production improves. Thus I might find myself harvesting 5 lbs of beans for every one a new gardener grows, just because my skills are more developed.
to single rows. )
Soil fertility and moisture regulation are big factors. Successful gardeners tinker with their soil fertility constantly and check moisture conditions often daily. I often see new gardeners checking their gardens once a week, and as interest wanes, even less.
So what about that broccoli? How many plants? Well I'll ask: which variety do you plan to grow? I often get blank stares in response. One response I liked,"Green." Ok. That's a start since broccoli comes in different colors. Different varieties of broccoli produce different sized heads under different growing conditions. Not one size fits all. Some varieties only produce one primary crown, then zip. Others will continue to produce smaller heads for some period of time. Others never produce a primary head, rather just lots of little ones. And the wrong variety seeded in the wrong climate zone will produce none.
Sticking with broccoli, the amount you get can be affected by the maturity at the time of harvest. If you let the heads break and start producing flowers, your traditional broccoli head is lost. That's not to say it isn't edible. A novice gardener might simply discard the plant. Not I. It gets eaten. Good for soup if nothing else. And how about the stem and leaves? You'll get a lot more food out of that broccoli row if you eat the peeled stems and use the leaves.
even though the plants start looking like miniature trees.)
How much zucchini? I'll ask, at what stage do you plan to harvest? Baby with flower intact? Gourmet size? 5-6" in length? Baseball bat size? Big difference in yield!
Harvesting lettuce and spinach at baby leaf size will cut into the total volume of your harvest. Harvesting one leaf at a time on older plants will slightly increase the amount compared to cutting the whole plant. So harvested amount depends upon lots of variables.
Of course we have not addressed weather, pests, disease. Lots of things can cause crop failure, or cut into the amount you get to harvest.
One big difference I see between my own gardens and those of new gardeners is planting strategy. Where a new gardener will plant a single row of bean plants 6" apart, I will put in a double or triple row 3" apart. Thus my yields are dramatically larger out of the bed even though each individual plant may be smaller. Where a new gardener will plant a row of cucumbers being trellised, I will sow beets around the base of those cucumbers thus getting a another crop to harvest. I see peas being planted in single rows, plants 4-5 inches apart. I will always sow peas in a bed, 2" apart in every direction. Same with radishes. I never plant radishes in a row. So my yields are much larger because I plant more in the same space.
Figuring out how much of what to plant is an individual thing. A lot depends upon what you like to eat. It makes little sense to plant 20' row of stringbeans if no one in your family eats them regularly. So if I tell you to grow 20' of beans for such a family, it would be a big waste.
So how big should your garden be? As big as you want it. I figured out how much I needed by starting out small and gradually increasing the size as time went by. I adjusted things as I went along. I still don't have it perfect, in fact I never will. I always grow extra of everything, that way I have surplus to trade, giveaway, feed to the animals. And when a crop fails, I'll have so etching else growing to fill in the slack.