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The grass pollen season runs from early May into September. The link is to a site of an anti-hayfever medication because it has a clear calendar with which pollen are a problem in which months, if in Dutch. I am not connected to the site and do not endorse the medication.
In the dunes there is relatively little grass and most of that is kept short(ish) by grazing animals. But the dunes are a relatively narrow area and if there are eastern winds you will be depended on whether the area is wide enough to protect you.
Almost all dikes in the Netherlands are covered with grass, as are many of the areas around it, cattle and sheep grazing and grass and hay harvest are common in many areas near to the dunes (if not all over.)
I think you will be relatively safe in Zandvoort, IJmuiden (even though it does not have that many places to stay,) Wijk aan Zee (though that has a big grass covered common on the land end of the village), in the one hotel on the beach at Castricum, Bergen aan Zee and Schoorl aan Zee. Note the 'aan zee' part of the names, the other parts of those villages/towns are often on the land side of the dunes and as such too close to the grass lands for your comfort. (I am not familiar enough with the towns more to the south, I know that those more to the north are close to grasslands.)
If you also suffer from tree pollen all of the coastal strip will be hard to impossible, depending on the weather.
I live in the area, in one of the towns just inland of the dunes and without hotels. I have a cousin suffering from grass pollen living just inland of the dunes and spring/summer was always a problem.
Upvote:1
It is unpredictable. The Netherlands like London lies in a zone where the weather is extremely variable. In addition the flat land leaves the country open to the winds coming from any direction. The seasonal changes may shift, some years you may have warm weather in April and a heat wave at the end of May, some years you may have in May wind chill so chilling that you might need also a scarf and a pair gloves.
If you can delay your decision until the very last moment you might keep track of the long term weather forecast. Otherwise you will have to take into account the uncertainty.
I do not know very much about Belgium, but given that also the Flanders have a flat landscape I suspect it is not different.