This is for the original proverb, which you've put as the question term. It's usually "la que calla es buena". What it means is that the best pears, like the best women, are "quiet", women because they keep their mouths shut and pears because if they're ripe, soft and juicy they don't make a crunching sound when you cut them open or bite into them.

"-LA MUJER Y LA PERA, LA QUE CALLA ES BUENA. La mujer por discreta y la pera porque no relincha al cortarla."
http://www.funjdiaz.net/folklore/07ficha.php?ID=1135

"La pera i la dona, la que calla és bona (Significa que la pera millor és la que és sucosa i blana)"
http://www.elrefranyer.com/ref/la-pera-i-la-dona-la-que-call...

This is not very PC, to put it mildly, and in English, literally translated as "quiet", it doesn't even have the saving grace of being a familiar proverb. Anyway, I'm not sure English-speaking readers would understand what a "quiet" or "silent" pear might mean. Maybe it would be best to put "soft" or "ripe", which is what it really means in relation to pears.

I admit, though, that it remains a sexist remark, and I don't know whether you would want to use it.

Anyway, your text apparently has "cala" instead of "calla". I don't know whether this is just a mistake or a deliberate variant. If they really mean "cala", and it's a wordplay on the original proverb, I would guess that it might mean "well-drained", since one meaning of calar is "permitir que un líquido pase a través de él". But in that case I have no idea how you would capture that wordplay in English; it really seems impossible.

This may be a case for a translator's note. Anyway, I think the above is an explanation, if not a viable translation.

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Note added at 1 hr (2014-09-22 08:51:18 GMT)
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Thinking about this further, since it's talking about the pear tree itself, I think "cala" is probably what they do mean, and that it is a play on words, since Spanish readers will know the version with "calla". So really (if I'm right) the message is "pear trees should be planted in well-drained soil", and frankly there's not much point in trying to render the original proverb, since it would not really be relevant.

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Note added at 1 hr (2014-09-22 08:56:37 GMT)
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Well, it's a view I've heard expressed ;) But seriously, if "cala" is right this is mission impossible.

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Note added at 1 hr (2014-09-22 09:17:47 GMT)
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Unless you make up a phony proverb of your own:

"Pear trees, like bathrooms, should be well drained".

As you said earlier in another place, tongue in cheek :)

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Note added at 1 hr (2014-09-22 09:36:06 GMT)
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Yeah, I'm not sure drainage is the issue. It was the only thing that occurred to me for "cala".
I like that one! It's still sexist, though, admittedly no more than the original, and in fact it's a bit less offensive, though English is less tolerant than Spanish in that area. Maybe there's a way of doing it without referring to women...