How do I sort and display by a matrix date field?
I have not worked with Lowblocks yet, but it looks like it returns an array of Matrix block models. To get to the related entries you need to access the owner property.

{% set entryIds = [] %}
{% for sown in allSown %}
    {% set blockId = sown.id %}
    {% set entryIds = entryIds|merge([blockId.owner.id]) %}
{% endfor %}


But do you really need those entry IDs? You could just display the blocks?

{% for sown in allSown %}
    {{ sown.owner.title }}
    {{ sown.sowingLocation }}
    {{ sown.sowingDate }}
{% endfor %}