Do Large Parks and Gardens Invalidate an Eruv for Shabbos?

Adapted from a shiur by Rav Yitzchak
Grossman on Parshas
Terumah

(אורך החצר מאה באמה ורחב חמישים בחמישים וקמה חמש אמות שש משזר ואדניהם נחשת (שמות כז:יח

The Gemara (Eruvin
23a) sees this pasuk
concerning the measurements of the Mishkan as an asmachta (allusion)
to the rabbinic halacha of a karpef: An area 5000 square amos that is lo hukaf
l’dirah (not walled for people to live) may not be included as part
of an eruv
to allow carrying on Shabbos.

It is based on the
following points:

• A karpef is similar to a reshus harabim (public domain where
  carrying is forbidden) in that it is open and not used for housing (Rashi).
• It is derived from the Mishkan like other
  halachos of Shabbos (Levush).
• Chazal only forbade a karpef larger
  than the Mishkan’s
  dimensions (Shulchan
  Aruch HaRav).

The Gemara states
that gardens with seeds (zeraim) planted in them are considered a karpef. If
so, how can municipal eruvin include parks and gardens? Many reasons for
leniency have been suggested:

• Parks planted for recreation are
  considered essential for living and are different than the Gemara’s case
  (Meiri, Maharsham, Nezirus
  Shimshon).
• Karpef is only when the wall (or eruv) was constructed first and then the seeds were
  planted (Divrei
  Malkiel, Sefer Poras Yosef).
• Seeds that grew by themselves (weeds) are
  permitted.
• Zeraim planted by non-Jews are permitted and cannot invalidate our eruv,
  plus it is no worse than if they grew by themselves (R.Shlomo Kluger, Divrei Malkiel).
• Does not apply to a city where the
  majority is urban and a small part has planted seeds (Dvar Shmuel,
  Chacham Tzvi). Moreover, it is part of the city planning and
  enhances it, so it wouldn’t be mevatel the dira aspect (Dvar Shmuel).   

These leniencies are
all subject to dispute, but over time the custom in many cities has been to be
lenient based on some/all of them when better options (e.g., enclosing the
area) are unavailable.