You have hit upon one of the major drawbacks of using landscape/weed blocker fabric - it's not very permeable. That fine textured plastic mesh (yes, it is woven plastic) simply does not allow water or liquid nutrients to pass through freely. It doesn't take a rocket scientist to figure out this may not be an ideal situation for your plants.......

And that is only one of a number of drawbacks of using this material. Seriously, most landscaping professionals and a growing body of home gardeners have determined that using landscape fabric is just not worth the time, effort and dollars involved in installing the stuff compared to any benefits it might provide. Other drawbacks:

1) It sucks the life out of the soil. Well, not literally but it contributes to the decline in soil biology. Healthy populations of soil organisms are absolutely necessary for the health and well-being of both the soil and the plants growing in it. These creatures must have free access to oxygen, water and organic matter to thrive and all that occurs at the soil surface. Covering the soil surface with a non-organic material like landscape fabric prohibits this free exchange. Just examine a soil that has been covered with landscape fabric for an extended period of time - 5-10 years. It will appear dead and lifeless - no worms or other visible soil creatures (not to mention the lack of the microscopic ones), no organic matter, soil will be pale in color and hard to dig, not loose, crumbly and rich.

2) Organic mulches - bark or wood chips, shredded leaves, pine straw, compost, etc. - feed the soil. Over time these organic materials breakdown and decompose, adding much needed nutrients to the soil. And they add to soil quality and texture, lightening heavy soils and providing some moisture retention to very fast draining, sandy soils. But they must have contact with the soil (and the organisms that inhabit the soil) to do so. The need to add any supplemental fertilizers is substantially reduced if one mulches routinely with a good quality mulch so your original question becomes moot ;-)

3) Landscape fabric hinders the development of any number of plants - expanding clumps of perennials, caning shrubs (most spring blooming types - forsythia, lilac, flowering quince, deutzia, beauty bush, etc. - even hydrangeas) and impedes the spread of rhizomatous or creeping, rooting groundcovers. And don't expect much from any self-seeding perennials or annuals. It is also harder to dig/divide plants surrounded by landscape fabric or to add new plantings.

4) Landscape fabric isn't nearly as efficient in controlling weeds as one might think. Annual weed seeds will still blow in and root easily in whatever is covering the fabric. Grass type weeds will easily grow through the mesh from underneath. And some pernicious, deeply rooted weeds - dandelions, thistles, rumex (dock), knotweed, horsetails, blackberries, ivy, etc. - will simply lift the fabric or seek out areas where the seams are accessible and pop up anyway. And it is far harder to weed through landscape fabric than it is to weed without it.

A properly thick layer (3-4inches) of the mulch of your choice will be as effective in controlling weeds as the landscape fabric will be and with none of the above drawbacks, provided you weed thoroughly before applying the mulch. Applying a pre-emergent herbicide - like Preen - before mulching will also help. Then all it takes is a weekly or biweekly sweep through the area to remove anything undesired that may pop up. How hard can that be?

Landscape fabrics are a pain in the behind. I do use them under gravel/brick/stone paths so that the sand/gravel stays separate. Future modifications become easier. Also good for lining the sides of raised beds to keep the soil in.

I think a little bit of discipline and a judicious choice of competing ground cover keeps the unwanted plants out of the yard more effectively. First couple of years are tougher. These are the things I do/do not:

a. do not leave behind any part of the roots.

b. do not let them go to flower.

c. keep the soil loose, it is easier to weed.

d. let sedums, purslane, clover, ajuga, low growing wildflowers and other low requirement plants take over. They are easier to pull out when needed. Mulch the rest.

e. do not till the soil too much. It destroys the natural eco-system and gives the weed seeds a head start.

I even weed 6ft into my neighbours yard as an added insurance. They do not garden much and they do not mind me doing that. I have never used a weed killer in the last 20 yrs of owning our house. As I said first few years were tough. Nowadays, I probably spend a total of 10hrs max in the entire season for weeding.